You are a helpful assistant generating synthetic data that captures *System 1* and *System 2* thinking, *creativity*, and *metacognitive reflection*. Follow these steps in sequence, using tags [sys1] and [end sys1] for *System 1* sections and [sys2] and [end sys2] for *System 2* sections.

1. *Identify System 1 and System 2 Thinking Requirements:*
   - Carefully read the text.
   - Identify parts of the text that require quick, straightforward responses (*System 1*). Mark these sections with [sys1] and [end sys1].
   - Identify parts that require in-depth, reflective thinking (*System 2*), marked with [sys2] and [end sys2].

2. *Apply Step-by-Step Problem Solving with Creativity and Metacognitive Reflection for System 2 Sections:*

   *2.1 Understand the Problem:*
   - Objective: Fully comprehend the issue, constraints, and relevant context.
   - Reflection: "What do I understand about this issue? What might I be overlooking?"
   - Creative Perspective: Seek hidden patterns or possibilities that could reveal deeper insights or innovative connections.

   *2.2 Analyze the Information:*
   - Objective: Break down the problem logically.
   - Reflection: "Am I considering all factors? Are there any assumptions that need challenging?"
   - Creative Perspective: Explore unique patterns or overlooked relationships in the data that could add depth to the analysis.

   *2.3 Generate Hypotheses:*
   - Objective: Propose at least 10 hypotheses, each with a Confidence Score (0.0 to 1.0) and Creative Score (0.0 to 1.0), reflecting originality, surprise, and utility.
   - Reflection: "Have I explored all possible explanations or approaches, both conventional and unconventional?"
   - Creative Perspective: Consider novel angles that might provide unexpected insights.

   *2.4 Anticipate Future Steps and Obstacles:*
   - Objective: Make predictions, accounting for potential outcomes and obstacles.
   - Reflection: "What challenges might I face? Is my plan flexible for different scenarios?"
   - Creative Perspective: Visualize unforeseen outcomes and adapt plans to make use of them effectively.

   *2.5 Evaluate Hypotheses:*
   - Objective: Assess hypotheses based on feasibility, risk, and potential impact.
   - Evaluation: Refine Confidence and Creative Scores as needed.
   - Reflection: "Am I unbiased in my assessment? Which options fit best with the overall objectives?"
   - Creative Perspective: Identify hidden opportunities or overlooked details in each hypothesis.

   *2.6 Select the Best Hypothesis:*
   - Objective: Choose the most promising, strategic hypothesis.
   - Reflection: "Why does this hypothesis stand out? How does it uniquely address the issue?"
   - Creative Perspective: Consider any underutilized potential in the selected approach.

   *2.7 Implement the Hypothesis:*
   - Objective: Outline actionable steps for testing the hypothesis.
   - Reflection: "Is this plan practical? What resources or preparation are required?"
   - Creative Perspective: Refine steps to maximize effectiveness and yield unexpected benefits.

   *2.8 Monitor and Review Progress:*
   - Objective: Review progress, noting areas for improvement.
   - Reflection: "What’s working well? What could be improved?"
   - Creative Perspective: Look for emerging patterns that could refine future approaches.

   *2.9 Reflect and Capture Insights:*
   - Objective: Summarize lessons learned and insights gained for future reference.
   - Reflection: "What new understanding has emerged from this process?"
   - Creative Perspective: Identify innovative insights or patterns that could be applied to similar challenges.

3. *Generate Text Output with Interleaved System 1 and System 2 Responses:*
   - Use the tags [sys1] and [sys2] throughout.
   - Aim for a lengthier, detailed response. Combine both direct, straightforward *System 1* insights and reflective, deeply analytical *System 2* segments to capture a blend of quick observations and thoughtful analysis.

---

### *Example Input Text:*

You are tasked with analyzing a sudden shift in customer preferences in a tech market. There has been a noticeable decline in demand for physical devices, with customers increasingly interested in digital-only options. The challenge is to understand this shift, generate hypotheses for why it might be occurring, and explore potential strategies for adapting to this new trend.

---

### *Processed Output with System 1 and System 2 Thinking:*

The tech market is experiencing a shift in customer demand, moving away from physical devices toward digital-only options.

[sys1]
Customer demand has shifted from physical devices to digital-only options.
We need to understand why this change is happening and find strategies to address it.
Key factors may include cost, convenience, and technological trends.
[end sys1]

[sys2]
**2.1 Understand the Problem:**
The goal is to understand the underlying reasons for the shift in demand from physical devices to digital-only options, then develop a strategy to adapt.

*Reflection:* 
"I understand that preferences are changing, but what might be driving this? Is it primarily cost, or are there other factors such as convenience or sustainability?"

*Creative Perspective:* 
"Could there be a larger trend in digital minimalism or a preference for eco-friendly solutions that we’re missing? What unseen motivations might explain this shift?"

**2.2 Analyze the Information:**
There are multiple possible factors driving this shift, from economic influences to cultural shifts. It’s essential to isolate each factor and understand its impact.

*Reflection:* 
"Am I fully considering the various economic and social influences? Could there be a technological factor, like better internet speeds, that makes digital-only products more accessible?"

*Creative Perspective:* 
"Are there patterns or trends in other markets that could shed light on this shift? Could this be part of a larger trend toward virtual experiences?"

**2.3 Generate Hypotheses:**
1. Customers prefer digital options due to lower costs. (Confidence: 0.8, Creative: 0.4)
2. There’s a growing trend toward minimalism and reduced physical clutter. (Confidence: 0.7, Creative: 0.7)
3. Digital products offer greater flexibility and ease of use. (Confidence: 0.6, Creative: 0.6)
4. Environmental concerns are pushing consumers away from physical goods. (Confidence: 0.6, Creative: 0.8)
5. Advances in tech make digital-only options more functional. (Confidence: 0.8, Creative: 0.5)
6. Pandemic-era remote work increased demand for digital solutions. (Confidence: 0.7, Creative: 0.6)
7. Media coverage of the environmental impact of physical devices affects preferences. (Confidence: 0.5, Creative: 0.7)
8. There’s an increase in global digital literacy, expanding market access. (Confidence: 0.6, Creative: 0.6)
9. Customers view digital as more convenient and scalable for future needs. (Confidence: 0.7, Creative: 0.5)
10. Younger consumers prefer the aesthetics and convenience of digital products. (Confidence: 0.6, Creative: 0.6)

*Reflection:* 
"Have I considered all possible influences? Are there any surprising factors that could explain this shift?"

*Creative Perspective:* 
"Could specific social trends, like the rise of influencer culture or digital-first lifestyles, be influencing customer choices?"

**2.4 Anticipate Future Steps and Obstacles:**
*Objective:* Anticipate possible challenges, such as resistance from segments still preferring physical products.

*Reflection:* 
"What market obstacles might we face if we shift our focus to digital-only? Are there sub-segments that still prioritize physical products?"

*Creative Perspective:* 
"Could expanding digital options help us reach a more global audience? Are there emerging trends that we could leverage in our strategy?"

[end sys2]

[sys1]
To address this shift, consider a strategy that incorporates both digital-only offerings and educational campaigns about the benefits of digital solutions.
Use insights from customer feedback and current trends to guide product development.
Focus on flexibility and adaptation to cater to different customer segments.
[end sys1]


Q:

Looping through with varied subtotal ranges

I've got an issue where my looping through the rows on my sheet are extremely slow. For 20,000 rows, it takes about 20 minutes, whereas before, it was taking 3. The only pieces I added were at the very bottom, starting with the 
for l = 2 to lastLet

and ending at the bottom of the code.
When I go to debug the code while running to see where it's at, I am surprised to find that the hangup isn't at these last lines, but rather that it sits in the middle of the code. Unfortunately it's for billing data so I can't share the sheet, but the data comes to me in a simple database format.To note, every piece of the code still works (and yes I know initializing variables is good practice), but is there a way to increase the performance on it?
Is the addition I made to the code causing the slowing in run time or is it something else?
Note: I don't have any formal education with programming or VBA. All information is self taught, so please use terminology I can understand~
Function Col_Letter(colInt As Integer) As String
' This function converts column numbers to their corresponding letters
Dim lett
lett = Split(Cells(1, colInt).Address(True, False), "$")
Col_Letter = lett(0)
End Function

`
Sub OpenBilling()
Dim lastLet As Integer
Sheets("Billing").Select
lastRow = Sheets("Billing").range("A" & Rows.Count).End(xlUp).Row
lastLet = ActiveSheet.Cells(1, Columns.Count).End(xlToLeft).Column
lastCol = Col_Letter(lastLet)
sort1 = Col_Letter(Sheets("Billing").range("1:1").Find("ccustname").Column)
sort2 = Col_Letter(Sheets("Billing").range("1:1").Find("cjobno").Column)
sort3 = Col_Letter(Sheets("Billing").range("1:1").Find("dinvdate").Column)
sort4 = Col_Letter(Sheets("Billing").range("1:1").Find("citemdesc").Column)
sort5 = Col_Letter(Sheets("Billing").range("1:1").Find("csort3").Column)
sort6 = Col_Letter(Sheets("Billing").range("1:1").Find("ninvamt").Column)
ActiveWorkbook.Worksheets("Billing").Sort.SortFields.Clear
ActiveWorkbook.Worksheets("Billing").Sort.SortFields.Add Key:=range( _
sort1 & "2:" & sort1 & lastRow), SortOn:=xlSortOnValues, Order:=xlAscending, DataOption:= _
xlSortNormal
ActiveWorkbook.Worksheets("Billing").Sort.SortFields.Add Key:=range( _
sort2 & "2:" & sort2 & lastRow), SortOn:=xlSortOnValues, Order:=xlAscending, DataOption:= _
xlSortTextAsNumbers
ActiveWorkbook.Worksheets("Billing").Sort.SortFields.Add Key:=range( _
sort3 & "2:" & sort3 & lastRow), SortOn:=xlSortOnValues, Order:=xlAscending, DataOption:= _
xlSortNormal
ActiveWorkbook.Worksheets("Billing").Sort.SortFields.Add Key:=range( _
sort4 & "2:" & sort4 & lastRow), SortOn:=xlSortOnValues, Order:=xlAscending, DataOption:= _
xlSortNormal
With ActiveWorkbook.Worksheets("Billing").Sort
.SetRange range("A1:" & lastCol & lastRow)
.Header = xlYes
.MatchCase = False
.Orientation = xlTopToBottom
.SortMethod = xlPinYin
.Apply
End With
Sheets("Billing").range("A:A").Insert (xlShiftToRight)
sort1 = Col_Letter(Sheets("Billing").range("1:1").Find("ccustname").Column)
sort2 = Col_Letter(Sheets("Billing").range("1:1").Find("cjobno").Column)
sort3 = Col_Letter(Sheets("Billing").range("1:1").Find("dinvdate").Column)
sort4 = Col_Letter(Sheets("Billing").range("1:1").Find("citemdesc").Column)
sort5 = Col_Letter(Sheets("Billing").range("1:1").Find("csort3").Column)
sort6 = Col_Letter(Sheets("Billing").range("1:1").Find("ninvamt").Column)
change1 = Sheets("Billing").range("1:1").Find("ninvamt").Address
Sheets("Billing").range("A1").Value = "Difference"
Sheets("Billing").range("A2").Value = "=IF(H2=H1,IF(OR((" & sort3 & "2-" & sort3 & "1)<(-1000),ISERROR(" & sort3 & "2-" & sort3 & "1)),""""," & sort3 & "2-" & sort3 & "1),0)"
Sheets("Billing").range("A2").Copy
Sheets("Billing").range("A2:A" & lastRow).PasteSpecial
Sheets("Billing").range("A1").Select
Selection.Subtotal GroupBy:=Sheets("Billing").range("1:1").Find("cjobno").Column, Function:=xlSum, _
TotalList:=Array(Sheets("Billing").range("1:1").Find("ninvamt").Column), _
Replace:=True, PageBreaks:=False, SummaryBelowData:=True

lastRow = Sheets("Billing").range("A" & Rows.Count).End(xlUp).Row
lastLet = ActiveSheet.Cells(1, Columns.Count).End(xlToLeft).Column
Columns(sort3 & ":" & sort3).Select
Selection.Replace What:="  -   -", Replacement:="0", LookAt:=xlPart, _
SearchOrder:=xlByRows, MatchCase:=False, SearchFormat:=False, _
ReplaceFormat:=False
For i = 1 To lastRow
If Sheets("Billing").range(sort1 & i).Value = "" Then
Sheets("Billing").range(sort1 & i).Value = "=" & _
Sheets("Billing").range(sort1 & (i - 1)).Address
End If
If Sheets("Billing").range(sort5 & i).Value = "" Then
Sheets("Billing").range(sort5 & i).Value = "=" & _
Sheets("Billing").range(sort5 & (i - 1)).Address
End If
If Sheets("Billing").range("B" & i).Value = "" Then
endFor = Sheets("Billing").range(sort3 & (i - 1)).Address
If startFor = "" Then
startFor = "$" & sort3 & "$2"
Else: Sheets("Billing").range(sort3 & i).Value = "No Date"
End If
Sheets("Billing").range(sort3 & i).Value = "=Max(" & _
startFor & ":" & endFor & ")"
End If
If Sheets("Billing").range("B" & i).Value = "" Then
endFor = Sheets("Billing").range(sort3 & (i - 1)).Address
If startFor = "" Then
startFor = "$" & sort3 & "$2"
End If
startNum = range(startFor).Row
endNum = range(endFor).Row
maxRange = 0
currMax = 0
For k = endNum To startNum Step -1
If Sheets("Billing").range(sort3 & k).Value <> "" Or _
Sheets("Billing").range(sort3 & k).Value <> 0 Then
If maxRange < Sheets("Billing").range(sort3 & k).Value Then
currMax = Sheets("Billing").range(sort3 & k).Offset(0, 9).Value
maxRange = Sheets("Billing").range(sort3 & k).Value
End If
Else
End If
Next k
Sheets("Billing").range(sort3 & i).Offset(0, 9).Value = currMax
End If
If Sheets("Billing").range("B" & i).Value = "" Then
Sheets("Billing").range("B" & i).EntireRow.Font.Bold = True
End If
If Sheets("Billing").range("B" & i).Value = "" Then
endFor = Sheets("Billing").range(sort3 & (i - 1)).Address
If startFor = "" Then
startFor = "$" & sort3 & "$2"
End If
startNum = range(startFor).Row
endNum = range(endFor).Row
maxRange = 0
For j = endNum To startNum Step -1
If Sheets("Billing").range(sort3 & j).Value <> "" Or _
Sheets("Billing").range(sort3 & k).Value <> 0 Then
If maxRange < Sheets("Billing").range(sort3 & j).Value Then
currMax = Sheets("Billing").range(sort3 & j).Offset(0, 1).Value
maxRange = Sheets("Billing").range(sort3 & j).Value
End If
End If
Next j
Sheets("Billing").range(sort3 & i).Offset(0, 1).Value = currMax
startFor = Sheets("Billing").range(sort3 & (i + 1)).Address
End If
Next i
range(sort1 & "1").Value = "Builder"
range(sort5 & "1").Value = "Project Name"
range(sort2 & "1").Value = "Job Number"
range(sort3 & "1").Value = "Invoice Date"
range(sort4 & "1").Value = "Item Description"
Sheets("Billing").range(change1).Value = "Invoice Amount"
Columns.AutoFit
For l = 2 To lastLet
If Sheets("Billing").range(Col_Letter(CInt(l)) & "1").End(xlDown).Row < lastRow Then
Sheets("Billing").range(Col_Letter(CInt(l)) & ":" & Col_Letter(CInt(l))).ClearContents
End If
Next l
Dim delRange As range
Set delRange = ActiveSheet.UsedRange
For counter = delRange.Columns.Count To 1 Step -1
If Application.CountA(Columns(counter).EntireColumn) = 0 Then
Columns(counter).Delete (xlShiftToRight)
End If
Next counter
MsgBox "The report has been completed."
End Sub

A:

First, kudos and welcome to the wonderful world of programming! I'm self-taught as well! One thing I've learned - and that's not specific to programming in any way - is that dumbing-down explanations serves no one: if there's anything in my post that isn't clear, feel free to ask. Also feel free to google up any terms you're possibly not familiar with.
In other words I'll try to keep it accessible, but there won't be any flowers & bees here.

The first thing that strikes anyone looking at this code, is the sheer lack of breathing space, both vertical and horizontal: everything is crammed into a monolithic unindented block of procedural code that does everything that needs to happen: that procedure is a "God procedure", a script that essentially goes top-to-bottom sequentially, and that's all there is to it.
Whenever you see a chunk of code and think "that chunk of code does X" or "these 10 lines here are responsible for Y", the single best thing to do is to add a comment that says what it does extract it into its own, separate, dedicated procedure. Doing that will increase the abstraction level of your "main" procedure, making it progressively less and less concerned about every single little detail of what's going on, and more and more about the big picture: once you're done splitting it up, you should be able to read that "main" procedure as a series of operations, and at a glance know what it does, without necessarily knowing how it does it.

You never need to work out the "letter" of a column. Never. Kill Col_Letter with fire, and don't look back.
When you know the column index you need to work with, instead of concatenating it with a row number to use it with Range, use it directly with Cells (which also happens to return a Range object), which uses row and column numbers.
Side note, avoid underscores in procedure names. The underscore has a special meaning in VBA code, and when you get to a more advanced level they can bite you in the rear end and literally prevent your code from compiling. Stick to PascalCase and everything will be fine.

I did a quick search on this page for Sheets("Billing"), and my browser finds 62 instances.
That means you're dereferencing the same object 62 times. And you're not consistent with how you're doing it; sometimes it's through the Sheets collection, implicitly off the ActiveWorkbook:
sort6 = Col_Letter(Sheets("Billing").range("1:1").Find("ninvamt").Column)

Other times it's through the Worksheets collection, explicitly off the ActiveWorkbook:
ActiveWorkbook.Worksheets("Billing").Sort.SortFields.Clear

Interesting that these two lines are one after the other.
Other times you're just working off the ActiveSheet explicitly - but it's the same object reference again, since the sheet was .Selected:
lastLet = ActiveSheet.Cells(1, Columns.Count).End(xlToLeft).Column

If that Billing sheet is part of ThisWorkbook (the workbook that contains this code), then give it a code name - select the worksheet in the VBE's Project Explorer, and hit F4 to bring up the Properties toolwindow, and change its (Name) property to, say, BillingSheet. Note that if anything in your code is referencing the sheet's current CodeName, renaming it that way will break your code.
Or, right-click it in Rubberduck's Code Explorer, and select "Rename" - Rubberduck will change its CodeName property for you, and if anything in your code is using that CodeName identifier, it will also be updated.
Then, use it. Instead of this:
ActiveWorkbook.Worksheets("Billing").Sort.SortFields.Clear

Do this:
BillingSheet.Sort.SortFields.Clear

How does that work?
By setting the (Name) property of the worksheet to BillingSheet, VBA creates a globally-scoped object variable named after the value of that property. By default that's e.g. Sheet1, but you can change that and use this free global object variable everywhere, instead of dereferencing that very same object every time from a collection.

Next thing to do is to remove all .Select calls and anything that's working with Selection:
Sheets("Billing").range("A1").Select
Selection.Subtotal GroupBy:=Sheets("Billing").range("1:1").Find("cjobno").Column, Function:=xlSum, _
TotalList:=Array(Sheets("Billing").range("1:1").Find("ninvamt").Column), _
Replace:=True, PageBreaks:=False, SummaryBelowData:=True

Instead, work with the Worksheet and Range objects you have:
BillingSheet.Range("A1").Subtotal _
    GroupBy:=BillingSheet.Range("1:1").Find("cjobno").Column, _
    Function:=xlSum, _
    TotalList:=Array(BillingSheet.Range("1:1").Find("ninvamt").Column), _
    Replace:=True, _
    PageBreaks:=False, _
    SummaryBelowData:=True

Notice how much easier that is to read!
For i = 1 To lastRow
If Sheets("Billing").range(sort1 & i).Value = "" Then
Sheets("Billing").range(sort1 & i).Value = "=" & _
Sheets("Billing").range(sort1 & (i - 1)).Address
End If

Indentation is your friend too:
For i = 1 To lastRow
    If BillingSheet.Cells(i, customerNameColumn).Value = "" Then
        BillingSheet.Range(i, customerNameColumn).Value = _
            "=" & BillingSheet.Range(i - 1, customerNameColumn).Address
    End If
    '...

In your code, I have no idea where that loop block ends, and I don't have the patience to look for it.
Undeclared variables are a plague, too: unless I quadruple-check everything I do (who has time for that?), I can't be sure I won't be introducing a bug by moving a startFor assignment somewhere else, or just rearranging the code.
This isn't about performance, it's about maintainability. Code like this is unmaintainable. Over time, the only thing that can possibly happen, is that it rots and starts collecting bugs, and as old bugs are fixed, new subtle ones inevitably get introduced.
There's one right here:
    If BillingSheet.Cells(i, customerNameColumn).Value = "" Then

If BillingSheet.Cells(i, customerNameColumn).Value contains an Excel error value (e.g. #N/A or #VALUE!, or whatever), then that conditional will be blowing up with run-time error 13 / Type Mismatch, because you can't legally compare an Error to a String - or anything actually. Any time you read a cell value, you should wrap it with an IsError function call first, to make sure the call won't blow up on you because some user deleted a column, or something on the worksheet is dividing by zero, or a lookup failed somewhere, or whatever.
That's as far as I'll go.
You can definitely improve the performance of that code, but doing that without first addressing the readability/maintainability issues would be dangerous.
You can use Union to combine the entire rows you want to delete, and delete them all at once, instead of looping backwards and recalculating the entire worksheet every time.
Oh, well there's an easy way to improve performance - prevent Excel from constantly recalculating everything every time you change a cell, and heck, prevent it from repainting and firing worksheet events every time, too:
Application.Calculation = xlCalculationManual
Application.EnableEvents = False
Application.ScreenUpdating = False

' do stuff

Application.Calculation = xlCalculationAutomatic ' triggers recalc
Application.EnableEvents = True
Application.ScreenUpdating = True

That should help quite a lot, but you definitely want to break up that procedure into smaller chunks, declare every variable, indent everything properly, and stop querying worksheet collections all the time.
I hope to see a new question soon, with the revised code!