I've conducted some testing. My rig is slower than Tony's and Acsscott so their results will differ. In PS24, using a 4k clip 50 fps, 1 minute long, I timed the preview rendering (brown bar turn to green) with the following results;
Timeline = 3840x2180 50fps; 3 min 20 seconds
Timeline = 1280x720 25fps; 3 min 14 seconds
Note; on my computer, anything more than zero for Playback Optimization would render the clip. I had it set at 5%
Now I conducted the same test using PS23:
Timeline = 3840x2180 50fps; 1 minute
Timeline = 1280x720 25fps; 40 seconds
Reducing the resolution while keeping the aspect ratio the same did improve the render marginally on my computer, the big difference is between the two systems themselves; PS24 vs PS23. PS24 is 3 times slower on my machine.