Modulo the necessarily sequential (as mentioned by rayiner), you are correct in terms of absolute speed achievable. But you achieve that speedup at a roughly linear increase in $$$ cost. When you have idle cores sitting around, maybe it was opportunity cost you were already paying, but that's bounded.

On the other hand, reductions in the amount of computation needed mean you get a faster build for the same resources, or a similar speed build for cheaper.

Depending on your circumstances, one of these things (speed vs. cost) might be much more important than the other. We cannot forget about either.