I've brought this up before, but most folks seem to assume that "Class 10" SDHC cards are better in all ways than lower classes. However, some brief internet searching seems to reveal that, generally, higher class levels only ensure faster speeds for reading/writing large files. For OS use or frequent use/movement of small/multiple files, lower speed level cards can often be far faster.
The guy in the thread you linked to was getting 50-100x quicker read/writes on small, random block access when using his Class 2 Sandisk card in comparison to his class 8 and 10 Kingston and Patriot cards. The latter two cards were clearly faster when it came to transferring large files (writing/reading images, audio files, etc.), but horrible when it came to "file system" use.
It may be worth doing a real speed test on your SDHC cards to determine how speedy they actually are.