I've got a 13 inch Macbook Air (4,2).
Earlier last year I tried to replace OSX with Linux (Ubuntu) and somehow managed to brick the machine, luckily the Mac store gave me a brand-new one. Since then I've just been running Arch through VirtualBox, and for a while everything was really smooth but there were a few hiccups, like power management not really working to the maximum.
Anyway lately I've started noticing that there's a bit of lag, not in boot time, but just general usage. I don't really have many things going on in the background, sometimes the Jekyll server when I'm working on my site, or just emacs with the SLIME REPL. The keyboard seems to respond a bit sluggishly and app switching is a bit slow.
I was wondering if anyone has the same setup and maybe have a few performance hacks that they would like to share? I'm not really familiar with OSX but the first thing that came to my mind was that maybe there were a few processes that were running amok without me knowing but the Activity Monitor doesn't show anything. Arch is definitely free of any runaway programs, and emacs with SLIME isn't that cpu intensive....