Here is an awesome speed run which sees Quake finished on Nightmare difficulty in 48 minutes. I always thought the original Quake was a lot longer than it really is. However, when it is completed in 48 minutes of course it seems very short!

Description: Rage through the 8 classic levels of sheer terror and fully immersive sound and lighting from the original Quake Shareware package. Arm yourself against the cannibalistic Ogre, fiendish Vore and indestructible Schambler using lethal nails, fierce Thunderbolts and abominable Rocket and Grenade Launchers. Frag monsters like in the old days with Quake Reloaded! Control [...]