The same applies when checking for changes on disk - right now git does not do that in parallel. These are low hanging fruit that I plan to pick for my own sake.
There is another contributor who is very interested in increasing pack performance, which will directly impact fetch and pull speeds. Judging from my experience with packs thus far, I believe a lot of options are still to be tapped in that field as well. Rust will open it up for contributions and experiments to a greater audience, so I would hope that this will go way beyond of what I can do.
Even though this project is not very contributor friendly right now, I will be working on improving this so more people can join in (see https://github.com/Byron/gitoxide/issues/8).