I have one specific Git repository here which is terribly slow when using SourceTree 1.6.11 but behaves perfectly fine when using 1.5.2. On 1.6 it's actually not possible to switch or create branches.

This effect is reproducible across three different machines.
The repo is hosted using Stash 3.3.1 our clients are Windows 8.1, we tried with system (1.9.4) and embedded Git.

Which information I can provide you with to help solving this problem?

The repository does have no untracked files and has two tags, three branches.