In SourceTree Settings > General. "Use Mac OS windows restoration" should be default if not the only option. Your default option is Restore windows on startup.

I do prefer to restore windows but your own window restoration is slow, I gain startup time increase as soon as I switched to the Mac OS windows restoration.

Mac behaviour can be changed by user every time when quitting app with option key modifier. Windows restoration also can be enabled or disabled in Mac OS System Preferences.