Are build systems typically CPU bound, even during heavy operations such as calculating dependency closure (or other things I'm unaware of)? If not, the I don't see a need to use a language that doesn't provide high level, somewhat costly, but easy to use and understand abstractions. And if it's faster than its competitors and it weighs less, then there's also nothing wrong with calling it "fast" and "lightweight" (though I have no idea whether please fulfills this)