Those old games could be so consistent because the software and hardware were both so incredibly simple compared to today.

Today, a PC game has to work on a large range of hardware that has come out over the past 5+ years. And there are GPU features that are only available on certain cards, like hardware ray tracing and things like DLSS and FSR for upscaling.

And the game engines are incredibly more complex today to handle modern expectations, with dynamic lighting and shadows, huge maps, etc.

It doesn’t matter what your standards are. Hard realtime just isn’t realistic or even possible any more, except maybe in a game that would be considered truly primitive by today’s standards.