It should be a bit more responsive. You could always back up all your stuff boot from the install disc (press and hold c at bootup with cd in the computer) and use disk utility to wipe the drive and reinstall.
My iMac works flawlessly with Snow Leopard. Did you do an upgrade or clean install? If you did an upgrade, try wiping the drive and then reinstall it to empty drive
Just some general questions with the purpose of identifying the problem:
Which model iMac would that be (when was it released)?
Have you updated to 10.6.2?
Barefeats has some interesting benchmarks comparing SL to Leopard, albeit with higher end GPU's. Maybe it's all driver related but along with all the other well documented issues, me thinks Apple rolled out SL too quickly. And the 10.6.2 update didn't do anything for system performance. Quite disappointing.