This is one of the most absurd things I've ever seen.

Single segment "deathless" run on Nightmare difficulty in under 44 minutes in-game time. It's apparently never been done before. There is technically one death in the run, but it's intentional and actually part of a trick that advances the map anyway.

And here I usually say "eff it" around map 11 on Hurt Me Plenty...

Impressive to say the least.  Speedruns have never been my thing.  I'm not happy unless I'm 100% kills, 100% items, 100% secrets.  That's not really possible on Nightmare courtesy of the monster respawning.  I think the only time I've done a speedrun was Shub's pit in Quake just to see if it was possible to rocket-jump to victory there.

I forgot to mention.  I did notice in the speed run that this guy does not know how to use the chaingun's first-shot accuracy to any good effect.  It was spray and pray the whole time.