When you talk about the speed of reading and writing small files, it's not the class that you're interested in.  The class only relates to the continued write throughput/speed on large write operations.  What you need to concern yourself with is the I/O operations measure, which unfortunately, is something that it's hard to get good data on.
Toms hardware have done a couple of SD card labs with the benchmarks that you need.   Though if you've got anything less than something like a Sandisk Extreme, it's likely that the I/O measure on your card is likely to be relatively bad.  Samsung's SD cards are good middle grounds in terms of price/performance on the I/O side, I run my OS off them.