I replayed Tomb Raider 1 and 2 recently and when playing them here in the Remaster (with original Graphics),
it somehow feels as it sometimes(!) runs even less than 30 FPS.
I also played the TR2 Demo on a original Win98, when i was in the Mood messing around with that Machine.
So my Memories are relativly fresh.

Is it just me having Fata Morganas, or is something behind it?

But i have to say, at least here on my Main PC my TR2 Copy has those Patches to play in 16:9.
Maybe that helped the Frame Rate? (But wouldn't explain my experience with the Demo on Win98)

Because i am curious i now also tried it out on PCEM and yes, it runs smoother.

I'm just guessing, but I think the original graphics are running in software mode, which even on modern hardware will still have some slowdown at higher resolutions.

It might be something else entirely though, maybe just some odd framepacing issue. The original games are locked at 30, just like in this remaster.