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.