If you have performance issues and use iTerm2 you might wanna disable the blur effect.
I manage to pinpoint this problem after a lot of frustration.
For more info see this: https://code.google.com/p/iterm2/issues/detail?can=2&start=0&num=100&q=&colspec=ID%20Type%20Status%20Priority%20Milestone%20Owner%20Summary&groupby=&sort=&id=1915
(this should probably not be classified as a bug, but there is no category for public service)
Customer support service by UserEcho