2) Linux forks significantly faster than anything else I know. For something like Chromium the compiler is forked bazillion times and so is the linker and nmake and so on so forth.
3) Linux, the kernel, is heavily optimized for building stuff as that's what the kernel developers do day in and day out - there are threads on LKML that I can't be bothered to dig out right now but lot of effort goes in to optimizing for kernel build workload - may be that helps.
3) Linker - stock one is slower and did not do the more costly optimizations until now so it might be faster because of doing lesser than the MS linker that does incremental linking, WPO and what not. Gold is even faster and I may be wrong but I don't think it does what the MS linker does either.
4) Layers - Don't know if Cygwin tools are involved but they add their own slowness.