It's probably not related to the speed and I am not entirely certain how Github stores the repository but I noticed Gitlab does something weird to the bare repository so it's not directly usable as a bare repository.

Gitea is an example I like because it stores the repository as a bare repository, the same as if I did git clone --bare. I bring it up because when I stopped running Gitea, I could easily go in to the data and backup all the repositories an easily reuse them somewhere else.