Same happens, or used to happen, if you add millions of commits to repo, each one referencing a different GitHub user number in the email address. It times out while building the contributor list to be displayed on the side column.
See related earlier submission (with a slightly confusing title):
To clarify, that earlier submission is about a likely contributing cause of the phenomenon reported here.
Shoot, I’ve seen it crash if you happen to have thousands of commits in the last month or two. Usually a refresh or two will make it slowly work, presumptively once the cache is warm.
Impressive that this is still happening a week later.