On my end, the system (2012 MBA, see etrecheck.rtf below for details) is laggy *without* having a high system load. As I mentioned above (
https://forums.macrumors.com/posts/20230908/), the system works fine for long periods of time until I load up Preview.app with a couple PDFs. I did an experiment yesterday. I had used my MBA for two full days running all sorts of applications (Safari, Firefox, MS Word, Numbers, VMWare View, iTunes, Mail, Calendar, etc.) with everything being smooth and responsive. Then, at the end of the day yesterday, I opened up 5 PDFs in Preview.app. Almost immediately, all animations became laggy. The lag persisted even when closing out of all running windowed apps.
Here is a snapshot of the system at that point:
https://www.dropbox.com/s/4124cyyzzi7ser5/etrecheck.rtf?dl=0
https://www.dropbox.com/s/m0ktnbykkk4btvg/Screen Shot 2014-10-28 at 3.56.51 PM.png?dl=0
https://www.dropbox.com/s/sakmx8g7wro8tbw/Screen Shot 2014-10-28 at 3.57.13 PM.png?dl=0
https://www.dropbox.com/s/423wci0154vd8fv/Screen Shot 2014-10-28 at 3.57.19 PM.png?dl=0
By all accounts, the system is idle. However, animations (e.g., switching desktops, etc.) are terribly laggy. Again, everything was smooth for two days prior to firing up Preview.app. I really do think the GPU drivers or WindowServer have a problem that Preview brings out.