Lower resolutions on smaller monitors. 256 colors at a time, no truecolor yet. Lower-res samples, if any. Lower framerate expectations - try playing Descent 2

Lower resolutions on smaller monitors. 256 colors at a time, no truecolor yet. Lower-res samples, if any. Lower framerate expectations - try playing Descent 2 on an emulated computer with similar specs to the lowest specs suggested on the box. Even one in the middle of the spec range probably didn't get a constant 60fps. More hand-tuned assembly (RCT was famously 99% assembly, according to Wikipedia; this was starting to be unusual but people who'd been in the industry a while probably did at last one game in 100% assembly, and would have been pretty comfortable with hand-optimizing stuff as needed). Simpler worlds, with simpler AI. Victory conditions designed to be reached before the number of entities in the game overwhelmed the CPU completely. Simpler models. Most 3d games you play now probably have more polygons in your character's weapon than Descent would have in the entire level and all the active entities; they certainly have more polys in the overall character. I mean, really, three million tiles? That's insane by that day's standards, that's a bit more than 1700x1700 tiles, a quick search tells me the maximum map size in Roller Coaster Tycoon 3 was 256x256, and it's a fairly safe assumption that RCT1 was, at best, the same size, if not probably smaller. I can't find anything similar for Sim City 2000 but I would be surprised if it was much bigger.