What lauguage are you typing in? There are some known problems with some languages such as discussed here: Debugging NSTextView performance problem when editing multiple languages
Unfortunately I don’t see any fixes on the near horizon. I’ve tracked the problem down into platform code that I’m not able to change. The performance problems that I’m aware of all all related to the amount of text visualy displayed in the text buffer, so you can mitigate most of the problems by focusing into a particular project, instead of viewing all of your projects at once.
Anyway please let me know if that seems like the problem.