The theory is simple: get a faster hard drive and your desktop computer boots faster, runs faster, saves faster, and opens files faster. That's one of the main reasons people buy drives like the VelociRaptor from Western Digital, which spins at 10,000 RPM. The faster the platters spin, the faster you can access your data.
Hard drives use rotating media, and we don't have those in our phones. We use solid-state media both for storing the OS (the ROM), and for supplemental storage (microsdhc cards). Just like hard drives which come in various speeds (4,200 and 5,400 RPM in laptops; 7,200 and 10,000 RPM in desktops) sdcards have different speeds, too.
How are Speeds Designated?
These speeds are ranked using a "class" designation. "Class 2" is arguably twice as fast as "Class 1" (or cards that don't have a class designation). Class 4 and 6 are "fast enough" for recording video, whereas Class 2 and below are just fine for storing data. Class 2 is what most newer Android phones are coming with.
Following with our desktop computer example, does replacing your slower sdcard with a faster one make your phone any faster? To answer that question I picked up the fastest card I could get my hands on, a 16GB Class 10 microsdhc card from Kingston Technologies.
Swapping out the 8GB Class 2 microsd card that comes with the T-Mobile G2 with the 16GB Class 10 was easy and painless. I didn't notice any difference in speed during the initial boot (faster or slower) or in general use of the device. Benchmarks didn't show any discernible speed difference with the new card versus the old card. Overall performance didn't seem to be improved either.
So, on a stock Android phone, will plopping in a significantly faster storage card make a difference in speed? In a word: no.
Why would someone want to buy a faster card? They're usually more expensive, so what benefits do you get?
First off, when I transferred my data from my Class 2 card to the Class 10 card in my desktop computer the data transfer time was significantly reduced (both reading and writing) when using the Class 10 card. Of course, that's not going to be a common use, but it's worth mentioning.
With Android 2.2 Froyo you now have the option of moving some apps from internal storage to external storage. Launching apps seemed to load in the same amount of time whether loading from internal storage, or either of the two sdcards.
Photos and Videos
Next, photos and videos default to the sdcard when taken using the Camera app. Snapping still pictures didn't seem any faster using the Class 10 card, but I noticed something interesting with video. Since most modern Android's can record video at 720p, the amount of video data that we're talking about is (to use a technical term) "a lot". You might be tempted to think that all this means is that you need a "big" sdcard to store it all on (and you're not incorrect about that), but that's not the whole picture.
When talking about video, you're essentially streaming a file to your storage card, very much like streaming a movie from Netflix to your web browser. Of course the throughput from your camera to your sdcard is a lot faster than your internet connection, but the concept is similar.
When you record video the data is stored in a buffer and streamed out to your storage card. The bigger the buffer (RAM), the cleaner the resulting video should look, until you're pushing data to the storage device faster than the device can write it. When this happens you get anomalies in the stream. These can be manifest by audio or video distortion, pops, blocky imagery, striping, banding, and other "stuff" that just makes the video a little less "high def".
That's the "interesting" part that I mentioned earlier. When using the stock Class 2 sdcard, video on my G2 always looked "pretty good" at 720p, audio was also "pretty good". When using the Class 10 card videos seemed just a little nicer, a little cleaner, a little more "high def" than with the Class 2. Unfortunately, despite numerous attempts, I couldn't get a demo that could clearly demonstrate this. You'll have to try it for yourself to see what I mean.
What about Non-Stock Devices?
If you are using a custom ROM that takes advantage of a swap partition (like some early CyanogenMod ROM's did), you'll notice a pretty significant jump in speed. The need for a swap partition was short-lived and isn't used much any more by Cyanogen's team. Other developers still swear by them and the faster the sdcard a swap partition is on, the better.
In the End
When all is said and done, is it worth it to replace your stock Class 1 or Class 2 sdcard with a faster card? Absolutely, yes.
Does Class 10 justify the additional cost over, say, Class 4? Not necessarily, but it does help videos look a little cleaner, and copying large amounts of data to and from the card via a card reader on a desktop computer much faster.
Would I buy the Class 10 card again? Currently the card that I bought is around US$92, including shipping. That's worth it in my book! Then again, I'm the guy that spent $120 on a 512MB MiniSD card just a few years ago.
What about you?
How fast is the sdcard that you have in your phone?
Have you noticed any speed differences from going to a faster sdcard?
How much extra (if any) are you willing to spend just to get a faster card?