Why fastDOOM is fast

How much faster is fastDOOM than regular Doom on a decked-out 486 from 1993?

30% faster without cutting any features! On a demanding map like doom2’s demo1, the gain is even higher, from 16.8 fps to 24.9 fps. That is 48% faster!

I did not suspect that DOOM had left that much on the table. Obviously shipping within one year left little time to optimize. I had to understand how this magic trick happened.
↫ Fabien Sanglard

What follows is an incredibly detailed exploration of why, exactly, fastDOOM is so much faster, by building and benchmarking every version, and even going git commit by git commit to really understand how fastDOOM’s developer, Victor “Viti95” Nieto, achieved these impressive results.