Fast compilation: less than a second (feels like not waiting at all)

Slow compilation: more than a minute (makes to start browsing HN, missing the end, thus losing even more time)

To have fast compilation even with big projects is hard. Go, C, and D are usually fast. Scala is usually slow.

I care about development builds primarily. The edit-compile-test loop must be really really fast. Optimization flags are irrelevant, because if performance matters you often must have them enabled for development as well.