No, not really. It's the phone's processor that determines speed, not the SD card size.
Incorrect, there have been several confirmed reports that faster read/write speeds on SD cards will result in a faster user experience when writing to the SD card, or accessing information on it (read).
Thus, you should have less lag browsing through photos, music, and utilizing apps if you're rooting and using an A2SD supported ROM.
The "size" of the SD card, to my knowledge, shouldn't result in any speed increase, just more storage.
Don't know which cards are the fastest overall on the market (I'd like to), but I've heard that SanDisk is generally regarded as the fastest (super expensive though).
I know a lot of people had seen some improvement when using a few Class 6 cards, but it's important to keep in mind that just because an SD card is certified to a certain "Class," that's just the MINIMUM read/write speeds. Speeds can actually be MUCH faster (thus, Class 4 SanDisks frequently outperform other Class 4 SD cards).
If anyone know which microSDHC truly has the fastest read/writes, that would be awesome.