In a previous post I complained that mr was too slow. madduck pointed me to the "-j" flag, which runs updates in parallel. With -j 5, my 11 repos update in 1.2s, so this is probably good enough to put this project on the back burner until I get annoyed again.

I have the feeling that the "right solution" (TM) involves running either git-daemon or something like it on the remote host. The concept would be to set up a pair of file descriptors connected via ssh to the remote git-daemon, and have your local git commands talk to that pair of file descriptors instead of a socket. Alas, that looks like a bit of work to do, if it is even possible.