Our system used to use grunt and then gulp and later webpack, and I was always annoyed at how slow the builds were. The startup time alone was several seconds,

Our system used to use grunt and then gulp and later webpack, and I was always annoyed at how slow the builds were. The startup time alone was several seconds, and it never used more than 1 core (our dev machines have 64 cores). I ended up rewriting the whole thing as rake tasks that call out to rollup and other tools. Now it's fast, and as a bonus I can actually understand what it's doing and why.