I do suspect this would not run well on a 75 Mhz Pentium 1. It would be very surprising if Quake 1 was actually the pinnacle of what as possible on the hardware of the time, though. id made exactly one game targeting that generation of hardware, and then their next game had meaningfully higher system requirements despite coming out only a year later. The hardware capabilities were changing so fast that there simply wasn't time to iterate on a specific target.
Not sure but I think it might have been getting close to what was possible - I remember reading about Michael Abrash working with John Carmack on all sorts of things to get acceptable framerates.
