I suspect there is a cacophony of work that happens when a commit hits the server. That request needs to get replicated, git repositories need to be repacked, pull requests need to calculate diffs, CI jobs need to execute, on and on.

That's also just assuming the good-faith usage. There are probably plenty of adversarial and poorly behaved scrapers that are putting additional load on the system.

Recalculate percentage of each language in the repo, recalculate top contributors, recalculate the stats for the committer's profile etc etc.