From: https://support.atlassian.com/servicedesk/customer/portal/1/STSP-5473

We've experienced some crippling performance issues using SourceTree as of late, where SourceTree will soft-lock for long periods of time (5+ minutes).

After some amount of effort we were able to get back to a working state by 

(a) Switching diff/merge tools from FileMerge to Beyond Compare

(b) Adding *.prefab and *.meta to the visual diff ignore list.

Different combinations of the above worked for different people.

Some context:

• We're using Unity to build a game, with about 15 developers.  
• We generate tons of commits every day with a large number of files, many of which are text files (e.g. unity prefab and meta files).  
• Only recently did SourceTree start giving us problems.  Rebooting/reinstalling/etc didn't matter.  Only when we changed the diff/tool and expanded the visual diff ignore list did things return to a workable state.