https://github.com/viti95/FastDoom
I'd like to point out this recent project that's attempting to optimize Doom (PCDoom by NukeYKT, a very recreation of the original DOS executables' source) and add further options to turn off/down some additional details/texturing to go even faster. It's already faster than doom.exe by default. It's mostly intended for the lower 486s and 386s (think about two digit MHz stuck on ISA video, around the 33MHz mark)
DISCLAIMER I DO NOT WORK ON THIS PROJECT. DO NOT MISTAKENLY REPLY LIKE IT'S MINE. I AM RELAYING THE EXISTENCE AND NEWS OF VITI95'S PROJECT