Theskytrooper

ZDoom hasn't been updated in years, but since it renders everything in software it's pretty light.

GZDoom does provide a software renderer but with all the stuff that has been added over the years the default settings could be taxing for your pc, so you should check the display settings and video mode.

Regardless, you should check out other sourceports if you're gonna play it vanilla. I recommend Crispy Doom.

thank you!