Yeah, but binary dependencies or watt-style precompiled macros aren't going to get improve the build times people really care about, incremental build times. The parallel frontend is the plausibly the last major improvement we'll see on that front for years.

Incremental matters more than clean build times because (A) you're likely to do a lot more of them (B) they break developer flow more than waiting on CI does (C) at least in theory, you can always add more cores to your CI and get reasonable speedups, less so for incremental.