Same issue with Lion and Mavericks, I have to turn it off or restart it in order for the issue to go away, it gets super slow randomly after a few good 20 minutes, it does not do it all the time, once i restart or turn off/on, it gets super fast like always. smcFanControl 43 degrees steady 699 rpm.
iMac A1225 EMC:2211 24/2.8/2x4GB(800MHz)/128GB SSD(Crucial M4-CT128M4SSD2)
iMac A1225 EMC:2211 24/2.8/2x4GB(800MHz)/128GB SSD(Crucial M4-CT128M4SSD2)