So they reinvented OpenStack's Zuul?

I've always thought if you're pushing changes at the sort of velocity that requires it, you're doing something wrong, anyway.

In my experience, it's a monorepo problem.

We have a monorepo at work and the CI only runs a minority of tests prior to merging, and then reverts bad commits later. It can take a few hours until every test has run including your commit. I don't know enough about the setup to know how it decides to run what tests though