Another helpful tool that should be mentioned in this context is the idea of merge trains, where a thorough test run is amortized over many commits (that should each have first received lighter selective testing).

This doesn't necessarily reduce the latency until a commit lands, though it might by reducing the overall load on the testing infrastructure. But it ensures that all commits ultimately get the same test coverage with less redundant test expense.

That avoids the problem where a change in component A can accidentally break a test that is only tested on changes to component B.

(It also eliminates regressions that occur when two changes land that have a semantic conflict that wasn't detected because there was no textual conflict and the changes were only tested independently.)