Summary

Everytime I give focus back to SourceTree (after working in another app) it hangs for over 30 seconds. While I'm working in SourceTree there are no performance issues, everything works nicely.

Environment

Windows 10

SourceTree 2.0.19.1 and later

Solution

Make sure you're not bookmarking repositories that no longer exist. 

When SourceTree gets focus again and wakes up, it restarts background processes that monitor the local filesystem in each bookmark and also query their remotes. Once you remove the non-existent repositories, this problem no longer occurs.