Card Speed . You will be much happier with your PDA / Smartphone if it is responsive. An SD Card that Writes / Reads fast is part of a good experience. Any file movement to and from the Card is affected. Applications that are data intensive, like audio and video, are most affected.
The Audacity applications are good examples. Audacity is a voice recorder. Audacity can be configured for several different audio formats, but if a traditional WAV file format is chosen, the data captured and resulting files can be very large. To save on internal memory, these files are usually written directly to the SD card. If you have a slow card, the data writing can't keep up and bits of audio are lost. This results is audio that sounds "fast and choppy".
The "Test" button performs 3 basic tests. Writing data in 32 bit chunks, writing data in 8 k byte chunks, and reading data in 8 k byte chunks. We recommend running the test four times. Make sure the results are consistent. Try the tests with an empty card and also with the card nearly full. Make sure you get consistent and good results.