I guess “purge 17% of the code” is not the correct answer?

If you can remove 17% of your code, or even just avoid building it, yeah you probably should. That can be a really rough tradeoff though. (And the sibling comment is also right that deleting 17% of code != 17% faster builds, though I could see it being higher or lower)

Only if your build scales linearly. Otherwise, it might be a different percentage.