I'm not sure why people are having slowdown issues with Original System nand 9.2, makes sense if its only GW as that would be dependant on memory card transfer rates.
Personally have had 0 problems with or without themes, either way it's nice and snappy. I'm using a 32GB Transcend Ultimate by the way. Benchmarks give a read speed of 83Mb/s and a write speed of 37Mb/s on an old macbook pro.
I just purchased a new SD card to test as well, a PNY Elite with about 90MBs read and 20MBs write. I'll post once I switch it out and see if performance increases. I forgot that I switched out the stock 2GB for a 16GB about a month ago and forgot to check what class it was (it was a freebie).