Think of it as a layered problem. If the bottom layer (CI) cannot keep up with the output of agents, then solving problems at a higher layer - like user experience checks - will be exponentially slower and less reliable. Kind of like how optimizing tight inner loops makes your whole program faster.

It's true because it usually makes no sense to QA test a build where the test suite has not yet passed.

Uh, no. If stage X is the bottleneck, it's immaterial how much you speed up stage Y.