If cloning a repo is an issue, you’re using CI wrong. --shallow has it’s purpose.

Anyway, in my project a complete CI run including local integration tests takes about an hour. We could cut that down by running things in parallel, but we never bothered to add more runners.

I would say, if your tests hold you back, you might want to reconsider testing. Staged testing is an option, or just reevaluate whether you really need all those test cases. Many integration tests are not really testing that much, because 95% of them overlap.