I would caution in thinking a higher class means the fastest performance. Yes, its R/W speeds are higher, but random access times usually suffer because of this. So for writing video, higher class the better. For loading apps or anything that needs to read AND write, thats not true.
Here is a good link on the matter if you want to read up. Note the Random 4K test, almost all higher class cards had horrible write times after reading.
32GB sd cards - xda-developers