Exactly. Multiply this with the time it takes for one run on a slow machine. Back in the day, I ran a compilation on my phone as it was the target device. Besides the compilation taking 40 minutes (and configure had missed a thing or two), the configure step itself took a minute or so. Because I don't know all the moving parts, I prefer start from scratch than running into obscure problems later on.

Arguing against parallelization of configure is like arguing against faster OS updates. "It's only once a week/whatever, come on!" Except it's spread over a billion of people time and time again.