[build2 author here] Thanks for the feedback! Some additional details:

> The first one is a criticism of CMake, not Ninja (?), so I don't think it can be why.

Fair enough. The point I was making is that if you want to compete with Ninja, you cannot leave any potential performance gains on the table.

> The second reason given is doing some work like header dependencies in multiple threads. This is the most plausible reason to me but it still feels unlikely.

We are talking about ~2% performance difference here. Parallelizing even a small amount of work across 24 threads rather that doing it serially saving a percent or two feels plausible to me.

> There is some hand waving about file access patterns but I am skeptical; if the end-to-end build time is 3 seconds then the project is small enough to all fit in kernel caches.

It fits into the system's file cache unless there is memory pressure, like one would expect from having 24 C++ compiler jobs running in parallel. We actually measured this in isolation (with more detailed results in the linked article) and it has a measurable effect.

> They also mention doing other things like invoking the compiler to get version information. This seems like it would dwarf any performance gain from number 2.

I measured this, it costs 70ms or ~2% of the overall time.