Q:

Slow SSAS cumulative sum calculated measure

I have a cumulative sum of a measure structured as:
Aggregate (
    { NULL : [Date].[Year - Month - Date].CurrentMember } 
    ,[Measures].[Applications] )

From the date of the first application to the current date, the days of the date range must be contiguous. 
The Date dimension however contains dates ranging from 1900-01-01 to well into the future. 
I've attempted to eliminate dates before the first application, and future dates by structuring the calculated measure as follows:
CREATE MEMBER CURRENTCUBE.[Measures].[Applications TD] AS
CASE 
WHEN   
    /* Eliminates dates before first applications, i.e. year 1900-01-01 */
    Aggregate (
        { NULL : [Date].[Year - Month - Date].CurrentMember } 
        ,[Measures].[Applications] ) < 0   
THEN NULL
WHEN
    /* Eliminates dates after today */
    [Date].[Year - Month - Date].CurrentMember. MemberValue >= StrToMember('[Date].[Date].&['+Format(Now(),"yyyy-MM-ddT00:00:00")+']').MemberValue 
THEN NULL
ELSE
    Aggregate (
            { NULL : [Date].[Year - Month - Date].CurrentMember } 
            ,[Measures].[Applications] )           
END

I have been unsuccessful in any attempt to optimizing this by aggregating only where needed using a SCOPE as an alternative to the case statement, utilizing EXISTS and EXCEPT functions and many others. 
When browsing the cube and dimensioning [Measures].[Applications TD] by [Date].[Year - Month - Date] user-defined hierarchy it is terribly slow. 

A:

IIF is generally faster than CASE, and SUM is often faster than AGGREGATE. 
Although your main problem is the second part of your condition using membervalue - is it required or will the following not do the same thing? : 
CREATE MEMBER CURRENTCUBE.[Measures].[Applications TD] AS
IIF(
     SUM (
        { NULL : [Date].[Year - Month - Date].CurrentMember } 
        ,[Measures].[Applications] 
     ) < 0 
   , NULL
   , 
   SUM (
         { NULL : [Date].[Year - Month - Date].CurrentMember } 
          ,[Measures].[Applications] 
    ) 
)

I'd separate this out as a custom member:
CREATE MEMBER CURRENTCUBE.[Date].[Date].[All].[Today] AS //<< a  little of syntax for this create 
    StrToMember('[Date].[Date].&['+Format(Now(),"yyyy-MM-ddT00:00:00")+']')

Then try a nested IIF:
CREATE MEMBER CURRENTCUBE.[Measures].[Applications TD] AS
IIF(
    [Date].[Year - Month - Date].CurrentMember. MemberValue >= [Date].[Date].[All].[Today].MemberValue
   , NULL
   , IIF(
       SUM (
          { NULL : [Date].[Year - Month - Date].CurrentMember } 
          ,[Measures].[Applications] 
       ) < 0 
     , NULL
     , 
     SUM (
           { NULL : [Date].[Year - Month - Date].CurrentMember } 
            ,[Measures].[Applications] 
      ) 
  )
)

BUT
Rather than bothering with the "Today" member it will be a lot more efficient if you add an isToday column to DimDate  - then have an attribute of the cubes date dimension using the column.  That way you should be able to simplify this [Date].[Year - Month - Date].CurrentMember. MemberValue >= [Date].[Date].[All].[Today].MemberValue

