Guys,
We tested a few SD cards and correlated the UI speed and the speed of which programs (like firefox) load with the minimum read rate of the SD card, suggesting that the file size transfers are very small for these operations.
Firefox load time on SD cards
Transcend (Class 2) 4GB - 9s
Sandisk (Class 2) 4GB - 10s
Kingston (Class 2) 4Gb - 21s
Transcend Benchmark

SanDisk Benchmark

Kingston Benchmark

Unfortunately many of the i.MX53 QSBs were shipped with the Kingston, but I would now suggest to use a SD card with higher minimum transfer rate such as the Transcend or SanDisk.
If anyone finds a SD card that performs even better then please post it here.