Quake II was a game made in an era of the AMD K6 and Intel Pentium I and II,and when hardware 3D acceleration was just about starting to be a thing - most people were still running it in software mode. It was designed to run at high FPS for the era. This is why it looks so primitive by today's standards and within a brief period games such as Unreal and Unreal Tournament raised the bar. Its not only the graphics,but the graphics design which is lacking,so polishing it up with better lighting,shadows and reflections is polishing a turd and for a game like that,its all about the FPS and exploiting glitches in the engine like the physics. TBH,I don't think making it run slower makes sense - I used to run games like that at lower settings to maximise FPS!