Then https://openwrt.org/docs/guide-developer/toolchain/use-buildsystem (and anywhere else git.openwrt.org is specified) should be updated to instead specify Github.
This has been a problem for over half a decade: Clone git.openwrt.org slow
Fortunately it looks like one can now log into the docs wiki with Github, we will see if I can fix it: 
Edit: fixed. Whoever made it possible to edit the docs with a Github account has truly benefited humanity. I will now fix problems wherever I notice them.