Repro steps:

Given I am Windows user with multiple git repositories open
When I switch between open repository tabs
Then SourceTree freezes up for over a minute

Solution:

Tools -> Options -> Un-check spell check commit messages.

Seems to me like you guys are possibly re-loading the spell checking dictionary or something any time the user switches between repositories. It's been painfully slow and entirely locks up the UI – not great for usability. Turning off spell check made repo switches instant vs. literally taking over a minute when I have 10 repositories bookmarked (3 open).

Easy fix? Disable spell checking by default in your next release and users will love you. I can accept typos in commit messages; I can't accept unusable slowness.