One thing to consider — one of the reasons I want our Actions CI to be fast is that I do a good deal of agentic coding work in parallel (worktrees) on very, very small VMs, but we have some computationally demanding tests and linters. Delegating the expensive validation work to a separate environment — the beefier Actions runner, with run-isolation — and then watching the check status actually tightens the feedback loop for these agents.

Like with all things CI, your mileage will vary according to where you write your code and what the code does.