What I have done - well, what I think I have done! - the UI is appalling - is added SourceTree.exe and git.exe as exclusions. My intention here is that the antimalware service won't then monitor their activity while they're doing stuff, but will still check working copies when it does a routine scan... but sadly this didn't seem to make any difference at all, literally none whatsoever, and SourceTree continued to be what I can only describe as "slow as fuck" - if you'll forgive the technical jargon.
The odd thing is, though, which just occurred to me like literally right now, it might actually be safer to add the folder exclusion after all! Because then there's zero risk of the Javascript interpreter springing into action :-|
What a time to be alive!
(But I still stand by everything I said about POSIX.)