I have a mid-2009 MBP currently running Mavericks and suffering from the slow performance problems that others have been mentioning.
Initially I thought it might have been something to do with a battery replacement warning I was receiving, so I had that replaced. Things got a bit better, but not by much. I'm planning on getting a SSD but that will take a while to ship.
The tech at the Apple Store suggested a clean install of Mavericks from a USB stick, which is what I'm going to attempt next. (if that doesn't work I'll probably try this as kernel_task seems to be taking up a lot of memory and CPU usage)
Is there a way to do a clean install but keep all of my apps, preferences, etc.? Could I just make a Time Machine backup and restore from it?