> I genuinely believe having fast builds is the most important part of the developer experience. Somewhere around a build taking more than 30 seconds is when bad things start to happen to my engagement loop. If I can keep it under 30, I can stay in flow the whole time. Other developers on the team have expressed similar tolerances.

This, a thousand times. Anything after 30 seconds is where you start to lose your focus, and your work output suffers for it.