I have a iMac 27 inch (purchased in late 2009). It has an Intel 2.66 i5 and 8GB of RAM. In the last 4 months, I have seen a big decrease in performance on the machine: endless rainbow wheels, hard drive sounds like it is thrashing, unresponsive scripts when visiting certain webpages with dynamic content, long boot times, etc. I couldn't find any runaway processes on the machine. 80% of the HD is free space, the CPU is typically at less than 20% utilization. One thing I did notice, is that minute after rebooting and logging in, roughly 50% of the memory on the machine is occupied--by what I am not sure. So I upgraded the machine to Mavericks last night: just the install took like 4 hours, and rebooting and logging in the first time, took over 1 hour. The machine was just thrashing endlessly. I've repaired disk permissions, but problems continue. Any idea what might be going on here? The upgrade just made things worse.