Nobody cares. Hashes are plenty fast. Even the code complexity doesn't matter: I've implemented both SHA-256, and SHA-512, take less than a hundred lines of code. And of course, Linus would have use some existing implementation.

Also, "more secure" doesn't necessarily mean "more complex", or "slower". Blake2b for instance is as fast as md5 and has a simple RAX (xor, rot, add) core from Chacha20.

It wasn't like that in 2005.

Which is largely irrelevant on desktops. Nobody's using git on embedded devices, so hash computational cost shouldn't be the deciding factor. Maybe people storing BLOBs will notice.

There are blobs in the kernel and speed was a very important motivation when kernel switched to git.

So now we need to assess whether hashes are a significant bottleneck.