What I've found is that if one of the files is very large then SourceTree become unmanageably slow. For example we use SCSS to compile a css file that is currently 436kb in size. Whenever changes are made to this file, SourceTree becomes too slow to use.
I think it's because it's trying to parse the entire file for changes each time. So it would be really handy if one could tell it not to show changes for a particular file and have it remember that setting.