Explore slow linux and macOS compile times

[rhuanjl]

The CI Linux and macOS jobs have always been much slower than the windows once. In the past I believed that was because the hosts used were slower.

BUT with the fix in #6592 the actual tests now perform at similar speeds - with linux sometimes being faster than windows, this strongly implies that the machines are not significantly slower.

However the compile time on macOS and Linux is still 3-4x as long as it is on windows - I'd expect PAL to add 10% - but the difference is so much higher than that.

Investigate why this compilation takes so much longer and see if it can be fixed.

Things that are not the answer:

• optimisation level (Debug which is not optimised and Release which uses -O3 take similar time to build)
• PAL compilation time - prior to Speed up linux and macOS CI by A LOT #6592 PAL was being built twice, cutting this only took off ~5 minutes online, maybe 2 minutes offline
• #pragma once the non-standard means for tracking included headers - I was wondering if this was failing and headers were being included multiple times BUT I've checked and that's not the case
• make vs ninja - testing offline difference was negligible +/- 8 seconds on a clean ~10.5 minute build
• gcc vs clang - testing offline gcc was actually 20% faster but that is again not the size of difference we're looking for