Good PC can't play 4k 120 fps footage smoothly even with proxys...

Your media is heavy long-GOP stuff. And your machine might not be able to handle both the playback along with all the other things an NLE has to do.

Just some relevant information here ...

Remember, Premiere isn't just playing back one file at a time, like a video player does. Premiere is grabbing bits and pieces of different files, to string them together on the sequence, plus add any changes ... effects ... to that played back image on the fly.

And long-GOP means that only every nine to up to 100 frames is an actual full image frame ... every "frame" in between is just a dataset of the pixels that 1) have changed since the last iframe or 2) will change before the next iframe or 3 ... BOTH.

So the computer, to display maybe 35 frames of one section, may have to decode and decompress up to 100 frames, fully processing them to memory, in order to create the frame you actually see. You may not even see the majority of the frames it had to create in order just to show you the ones of that sequence.

I work for/with/teach pro colorists, mostly based in Resolve or Basellight, and working on machines that make yours and mine look like toy computers. They HATE long-GOP media with a purple passion. And many still when going through the media delivered to them to grade, on finding any  long-GOP, immediately make transcodes to use while grading.

They don't want the hassles and struggles inherent in long-GOP media with heavy effects applied.