Hopefully it is not a coincidence that Steve Streeting left the SourceTree team four years after the first SourceTree release and shortly after the first release of SourceTree 1.6. version, but the SourceTree 1.6.x versions suck: it often hangs and is (often extremely) slow. This apart from numerous NullReferenceExceptions. I find this odd, as 1.5.x is far more stable in that respect, hence my hope it is not about Steve Streeting.
Even though many people complained about 1.6.x. being bad (read the comments on the release page), and quite a few ask for a way to downgrade, a downgrade option isn’t provided there.
A SourceTree downgrade itself is not that difficult. Ensuring the embedded versions of Git and Mercurial/Hg are up to date is more difficult. Getting that right is especially important on Windows: hgflow works much better with the embedded Mercurial/Hg. More importantly: both Git and Mercurial/Hg recently had a very nasty security issue.
So here are steps on how to downgrade, then upgrade the embedded versions of Git and Mercurial.
The Wiert Corner - irregular stream of stuff 