Is your iMac a early white 2007 model or mid to late silver 2007 model?
How much free space on the HD is left?
How much RAM are you running?
Have you tried repairing permissions using Apple's Disk Utility?
Try using something like OnyX to clear out a lot junk in your system?
If your iMac has only the minimum RAM it shipped with, you may need to increase your RAM to the Max. amount your model iMac can take.
The 2007 white iMac can take 4GBs of RAM, but is only cpapble of addressing 3 out of the 4GBs.
Later 2007 iMac models can take a Max. of 6GBs of RAM. Installing the Max. amount of RAM will help, somewhat, with your issues, also.
Another option is to downgrade your OS to OS X 10.6 Snow Leopard.
You'd have to backup your data and do a clean install of OS X 10.6.
I assume you have SL discs becuase the only way to upgrade to Lion was if you had SL installed.
Still following my advice, SL may run much better for you given the age of your iMac.
I have been away for a few days; sorry for the delay in responding back.
Thanks for your suggestions. I have done a few of them: increased RAM to 4G; repaired permissions; and used not Onyx but CleanMyMac to try to clear junk. I have the later 2007 iMac, but I didn't know it could take 6 G RAM.
I'm not sure I want to try going back to 10.6 at this point. Your suggestions have been helpful in that it tells me I was on the right track. Actually, I have noticed some improvement, hopefully enoough to get me to the new iterations of iMac this September.