I've been having this problem for a while now and have finally gotten around to trying to figure it out.
I've got a 3 year old Pangolin Performance, panp4n. For about a year or more it's seemed to have gotten slower. Whenever I shut it down or restart, there is some text that shows up for the briefest instant about shutting down a running process. I'm assuming that something is running in the background slowing stuff down but when I look at the system monitor I don't see any offenders. I suppose I may not be looking for the right thing, but I don't see anything hogging the cpu.
Any thoughts on what I should try?