Large CPU and memory fluctuations after simple Alt-Tab switching to SourceTree

My steps to reproduce

1. Launch SourceTree
2. Open a large git repo with lots of branching and long history
3. Switch the active application to something other than SourceTree
4. Wait for SourceTree.exe to go idle (<1% cpu usage in Task Manager)
5. Switch back to the application

Exepected behavior

• Application would go idle very quickly, and there would be little change in over all application memory usage

Actual behavior

• Cpu usage of application of the application takes up one of my cpu threads (13-15% on an 4 core / 8 thread CPU) for almost 30-40 seconds, while memory usage will increase and decrease by as much as 400-500 MB before the process completes.

As noted, this seems to be highly dependant on the repository which is open.  When i close my main (large) project and switch to something with very little branching or a smaller commit history, this problem appears to largely disappear.

During this period, Windows will frequently mark the application window as unresponsive periodically for a second or two (the standard windows title bar, and red x render over the blue window bar).  The application itself responds to mouse clicks very erratically or slowly.