My first guess would be that git is going to be faster since it won't be doing a huge number of stat() calls and similar to grab metadata about the files it's c

My first guess would be that git is going to be faster since it won't be doing a huge number of stat() calls and similar to grab metadata about the files it's creating, instead git will have the data in a single data structure it can efficiently ask for it from. Of course the filesystem has this too, but git will have it in a single location and probably more compactly, leaving it able to do it all with fewer reads. Then it has the data in the files compressed so the read is faster too. All this should mean that git can get to writing the files faster than cp will.