Most of this email thread is comparing Mercurial (Python) to Git. I'm not familiar enough with Git's internals to know why and where the languages are split, but it uses a significant amount of Shell scripting and Perl it its code base. You can put 'git-foo' anywhere on PATH and it'll get picked up. So in the comparison they're making startup time doesn't seem to be an issue for a combination of those languages, but it is for Python. It doesn't sound like their problem is that Python is an interpreted language.
Git is actively rewriting many of their shell and Perl code in C. Performance and portability are given as reasons (having shell and Perl as a dependency on Windows is a bit of a kludge). And shell scripts are much slower on Windows because new process overhead on Windows is ~10x what it is on POSIX platforms. (New threads, however, are faster to create on Windows than on POSIX.)
