I have a 2009 Macbook Pro, 4GB RAM, 2.26GHz Intel Core 2 Duo. Check battery warning but nothing else overtly wrong.
Since I updated to Mavericks (from Snow Leopard), it has been running incredibly slowly and the fan has been spinning up constantly. Icons bouncing for a long time in the dock before launching (especially Skype), really slow to type in the new Pages (old one is fine), etc., despite Activity Monitor showing low or medium memory pressure and the CPU being at relatively low usage.
Resetting the SMC brings it back to the speed it used to be at, but after a couple of hours of usage, my MacBook has slowed to a crawl again and the only thing that will speed it up again is another SMC reset.