/*No rant*/
Is a Pi3 good enough as Git Server?

should be, depends how much storage you have on it....

Not a problem

It won't create any additional lag?

I haven't been using git a long time but from what I can tell, git uses SSH to communicate and transfer data.... and ive moved literally a clone copy of all the data of the pi onto my pc no problem in terms of back up... it might be a little slow but for files less than 512mb, should be a breeze.... you'll be fine...

you also have to consider how much people will be using it bit it should be a problem....

Concurrency and IO will be your blockers. If you plan on running a layer on top of Git like gitlab, then I wouldn't recommend it.

If it's only Git that you'll connect using SSH, then you can run with it

Hahah no no gitlab :D

I run my git server with a B+ and it works like a charm 😉