The best CPU this series had was the i7 2.8 GHz. There really isn't anything better that will work in your system.
How about focusing on doing some house cleaning on your hard drive instead. A slow or sluggish system is one that needs some drive TLC!
- Clean out all of your old stuff you don't need any more (back it up)
- Clear out the old caches & log files. I find this tool makes quick work of it: Disk Doctor
- Run Disk Utilities to check your drive (from an external bootable drive) to make sure the drive is healthy
- Either scrub the drive down (back it up & reformat it, then restore your stuff back on. Or better, use a good drive defraging tool like this one: Disk Genius.
The last thing you might want to consider is to install a second drive (SSD) in this case make it your boot drive and leave your old internal as the data drive. Here's the IFIXIT guide to do this: Installing iMac Intel 27" EMC 2309 and 2374 Dual Hard Drive.
And the last possibility here if you don't want a dual drive setup is to upgrade your HD to a hybrid drive (SSHD). Here is what I would put in: Seagate Desktop SSHD. You'll also need this: OWC In-line Digital Thermal Sensor for iMac 2009-2010 Hard Drive Upgrade and lastly, here is the IFIXIT guide: iMac Intel 27" EMC 2309 and 2374 Hard Drive Replacement.