(I work on Git in my copious free time)

One of the reasons git-rev-parse takes slightly longer than your implementation is that you just unconditionally truncate the SHA-1 to 10 bytes. E.g. run this on linux.git:

    git log --oneline --abbrev=10 --pretty=format:%h |
    grep -E -v '^.{10}$' |
    perl -pe 's/^(.{10}).*/$1/'

You'll get 4 SHA-1s that are ambiguous at 10 characters, this problem will get a lot worse on bigger repositories.

Which is not to say that there isn't a lot of room for improvement. The scope creep of initialization time is one of the things that tends to get worse over time without being noticed, but Git unlike (apparently) Python makes huge use of re-invoking itself as part of its own test suite (tens of thousands of times), so it's naturally kept in check somewhat.

If you have this use-case I'd encourage you to start a thread on the Git mailing list about it.