Ingo Molnar spent a year trying to improve build times for the Linux kernel [0]

And that was for C, which is relatively straightforward compared to c++.

I suspect it would take months to sort out the project I am on now, and even then full builds would still be slow.

To suggest it could be fixed in hours is ignorance.

0: https://lore.kernel.org/lkml/YdIfz+LMewetSaEB@gmail.com/T/#u