As issue 2497 explains, svnserve waits for all child processes of the post-commit hook to finish. This means that if you background a job in your hook, expecting that to make the commit finish faster, you'll be unpleasantly surprised -- commits will take just as long as if you hadn't backgrounded the job. The solution is to redirect stdout and stderr of that backgrounded job. So: it'd be nice if the post-commit.tmpl _said_ that in one of its many comments.
Original issue reported by offby1