> I would think/guess you weren’t fully building from scratch, but from intermediate snapshots. If Linus pushed a commit, would your full build, minutes later, use it?

Yes, I would frequently download e.g. new kernel release tarballs (this was before Git) and slot it into the system. This didn't require recompiling anything but the kernel. Actually installing Stage 1 Gentoo required compiling everything (although it was on top of a compiler binary for bootstrapping.)

My hardware was cheap 2001 era consumer hardware, so I doubt it was that much faster than what the Windows developers had available. Besides, my question is more about why Windows (or anything else) would be difficult to compile, rather than just time-consuming. The nice thing about recompiling an entire operating system from scratch is that there are no external dependencies, because you're building everything! (Except the bootstrapping compiler, but for the Windows operating system there's no reason to rebuild that.)

Back in 2005 or so i was building freebsd, x11 and kde from scratch without issue. But these os’ are better engineered as there are no managers getting in the way.