git usually is lightning fast, it even knows how many processor threads are available and uses them. for a test i converted https://gar.svn.sourceforge.net/svnroot/gar/csw/mgar/pkg into a git repository with one subrepository for every directory. rebasing takes now 3 hours, while git without subrepositories takes 10 minutes.
$ ls -1 | wc 1162 1162 9527 $ time git submodule foreach git svn rebase .... real 199m29.626s user 60m49.631s sys 70m51.888s
contrary, if one creates one git repository for all these little directories, rebasing is twenty times faster: $ time git svn rebase ... real 10m1.032s user 5m33.632s sys 2m53.955s
rupert