You three must have been playing a completely different Doom to everyone else (unless you're all playing on "I'm too young to die" difficulty or quit after the first 5 levels), as the originals did indeed have literally dozens of enemies onscreen at once which didn't grind a 486 to a halt. Maybe not "100" simultaneously, but having recently replayed them via source port GZDoom, there certainly were up to 20-40 onscreen at once on a regular basis on many levels (and often over 50 in large chambers / outdoor areas once multiple Pain Elementals started each spitting out multiple Lost Souls at a rapid rate on Nightmare mode (or Ultra Violence + "fast" switch)):-
http://i.imgur.com/yfGNnax.jpg
And immediately behind you:-
http://i.imgur.com/sDF1CQh.jpg
And that's just level 8 in Doom 2 ("Tricks and Traps") during the 'easy' 1/3rd of the game where there are something like 80x enemies in 4x rooms back to back. Later levels were harder, Final Doom (Plutonia & TNT) were harder still, and many community WADS even harder than those (and some of those had monster counts of over 200 or so in one room / arena).
"Doom wasn't about literally dozens of enemies coming at you at once in the same room". No, really, it was.