I'm afraid I still don't understand. One factor is having fewer features and not looking for obsolete files, that I can understand. I guess the other thing is using better rules to figure out when a file truly need to be rebuilt?
To be honest, it's not clear to me why other systems are not faster. Ninja is relatively straightforward but also not too clever.
Now that I think about it, I did write more about some of the performance stuff we did here: https://aosabook.org/en/posa/ninja.html Looking back over that, I guess we did do some lower-level optimization work. I think a lot of it was just coming at it from a performance mindset.