You know, I usually say 3D Realms made the 2 best FPS EVER in history of gaming: Duke Nukem 3D and Shadow Warrior.
Both games are fast, ask for good reflexes, are simple to play, have balanced and interesting weapons and items, and are a big blast in multiplayer mode (Much better than Quake, Unreal, Counter Strike or anything else that's "cool" today). Their level editor is also quite easy to use if you want to do simple levels, and it gets more complex only if you want to do more complex levels.
The only FPS to get close to them is Serious Sam, but I didn't play Serious Sam that much to be able to tell how good it is. I also didn't try Serious Sam multiplayer mode, so I can't compare it with Duke Nukem/Shadow Warrior multiplayer mode.
It's unusual to see me praising an FPS, but I really enjoy those 2 games. If you never played any of those 2 games, give it a try (Except Twistin Ghost and Akira, who I know wouldn't try them even if God recommended them
) . Behind the crude graphics for today standards, hides the best FPSs ever done IMHO.
Oh, regarding speed issues on Duke 3D... I once made a level for it that CRAWLED in my P233. In my mate's Duron 800 it runned in just an ACCEPTABLE speed, not at full speed. I had to edit it a lot to make it run in a decent speed, and it ended up being a lot different of what I wanted to do.