[HTC Pico (Explorer)] mmk.dr:
Ok people, firstly, forget the thread title. Wrote it that way just to catch your eye :p

Anyways coming to the point. I've seen in many threads (especially Sense Rom threads) about questions and discussions about Size and Class of SD card.

They were all about buying cards with higher class for better speed and for providing a lag free experience.

Even I was considering buying a new one, so did some research [by research, I mean lot of Googling ;)].

Decided to share what I learned and in the process help you in your choice.

WHAT ARE SD CARDS?

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Well SD stands for Secure Digital. It is one of the formats of Memory Card.

This format includes 4 types of cards.

The Standard Capacity (SDSC)

The High Capacity (SDHC)

The Extended Capacity (SDXC)

The SDIO (which combines input/output functions with data storage)

And these types come in basically 3 different sizes - Original, Mini and Micro.​

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Since now we know what are SD cards, let's move on to the Speed Class Rating of a SD card.

What does the Class of an SD card reveal?​

We usually talk about the speed of cards in terms of 'Read' and 'Write' speeds.

But while assigning a Class to a SD card, only the Write speeds of that particular SD card are considered.

With SD cards, manufacturers generally rate the cards in three different ways.

The first method is through the use of Speed Class Ratings.

For example, most SD cards will be rated at Class 2, Class 4, Class 6 or Class 10.

The number denotes the minimum sustained Write speed for an empty card. So real-time speeds generally vary and are usually higher.

The second method is a newer method of the class ratings called UHS (Ultra High Speed). UHS-I cards are much, much faster and can support up to 104 MB/s.

The third method is through the use of “x” ratings. For most computer enthusiasts, this should be quite familiar since it’s just a multiple of 150kB/s, the speed of the standard CD-ROM drive (Oh yeah, that old thing…). Therefore, an SD card with a 133x speed rating should be able to achieve a transfer rate of 19,950 kBytes/s, which is about 20 MB/s. High speed SD cards will sometimes use the x rating as class ratings end at Class 10.

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However there's a catch.

The Write speeds used to denote a Class are "Sequential Write Speeds". Well you might ask, whether it matters to a normal mobile user.

The answer is maybe or may not.

Here's why: The Sequential Write speeds or Sequential Access is the speed with which large amount of data of a SINGLE FILE is written.

For example, while transferring data from system to your phone or vice versa, or when a digital camera records large photographs or videos.

However, for a smartphone user, there is another important speed to be considered - "Random Access". This is writing a large number of small files. Random read-write is the most important for speed during normal phone performance.

One study found that, in this Random-Access use, some Class 2 cards achieved a write speed of 1.38 Mbit/s, while all cards tested of Class 6 or greater (and some of lower Classes) including those from major manufacturers, were over 100 times slower.

By saying that,

I'm not suggesting that all Class 2 SD cards are good for your phone.

You might wonder, why does Higher Class cards get lower Random Access readings?

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GEEK ALERT:

Before you read further, you must be a geek or newbie who is greedy to grab everything that's thrown at him.

If you're just looking for the nutshell, ignore what's given below.

Given below are bunch of benchmarks that usually are to be taken into consideration, rather than just "Read Speed and Write Speed"

Sequential Read

Sequential Write

Random Read 512KB

Random Write 512KB

Random Read 4KB (QD=1)

Random Write 4KB (QD=1)

Random Read 4KB (QD=32)

Random Write 4KB (QD=32)

How to benchmark your card?

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Benchmarks are a rather elusive science since there are too many variables that can impact the results, not the least of which is the card itself. Anyone who has worked with SD cards in the real world has probably noticed that the brand of card can make a dramatic difference in performance. Tests have shown as much as a 12x difference in performance between best and worst cards in real world tasks. As already noted, the "class" ratings used by manufacturers as a measure of performance are not reliable indicators of performance either. In fact, some manufacturer's class 2 and class 4 rated cards routinely outperform cards rated as class 6 or class 10!

Consistency of testing is important since different devices and bottlenecks, such as bus speeds, buffers, file systems and activity can all impact and alter the results. This is why a card that has performed extremely well on a test may turn out to be a dog in your device. Short of testing all cards in a lab environment under exacting circumstances may well yield inconsistent results.

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There are a lot of apps available for bench-marking, for Android, as well as Windows and Linux.

Bench-marking Tools​

CrystalDiskMark

HD Tune

AndroBench

Others such as SD Tools, Antutu etc.

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Few Examples that might help:

Now we come to the question - Which SD cards should we go for?

Well, that's upto you, the end user, to decide based on your requirements.

On my part, I've provided you the information to help you with your decision.

Yet, for those who don't like to take decisions, here's something which might help.

Still confused?

Well I'll let those who already bought new SD cards to post their reviews.