Yeah, it basically has to advertise everything it has, so if you have a lot of references, it can be a quite large exchange before anything is done.
It seems like large GitHub repos get an ever-growing penalty for GitHub exposing `refs/pull/...` refs then, which is not great.
I will do some further digging and perhaps reach out to GitHub support. That's been very helpful, thanks Scott!
> An immediate benefit of the new protocol is that it enables reference filtering on the server-side, this can reduce the number of bytes required to fulfill operations like git fetch on large repositories.
[1] https://github.blog/changelog/2018-11-08-git-protocol-v2-sup...