It seems Murky constantly refreshes status on the repository. On very large repositories each refresh can take many seconds or minutes where the application pinwheels.
Events that seem to cause Murky to refresh: - When Murky gets focus. - When pushing the "Commit" button. - After a commit dialog says there are other files to be added and you push Cancel. - Some times when selecting files by clicking on them.
Needless to say it's annoying to have to keep focus on Murky while it's refreshing lets it lose focus and when you go back to Murky it just wants to refresh again.
It seems like a prudent way to deal with this is to not automatically rescan ever and just when the user selects it from the menu. Possibly add a rescan button next to Commit/Commit All.