Seems like corel should release a patch to roll back the last patch, until they figure out what they did.

What version of ps24 was installed when you reinstalled.  My guess would be the latest version equivalent to the downloaded patched version 24.1.0.260, which has the problem.  You should ask them how to install the previous version, or restore from the backup you made before installing the latest patch.

1080p on my computer is no delay.  I set my render threshold to 10, so only a few transitions have to bg render.  No complaints with timeline speed. 

The aspect ratio of 3840x2160 is the same as 1920x1080?  Timeline resolution has nothing to do with the export resolution.

A different timeline aspect ratio than export ratio will make changes to titles and effects.  I have no issues between 4k and 1080p. 

Not only can you set the keyframes, you can click on the lines between the keyframe markers and adjust the transition.  It also auto saves your edit so when you open the clip again it asks if you want to use the previous.

I use insta360 studio to make my 2d clips, then use those clips in ps.  The insta360 studio 3d to 2d is SO  GOOD!  Best I've ever used.  I use it with all my 3d cameras.

The timeline settings have nothing to do with the output resolution or quality of the export.  As long as the export aspect ratio is the same as the timeline aspect ratio, you're good.

That is what i do.  I use 1080p timeline settings for all my 4k projects.  You don't need to change the timeline to 4k before export, just export at 4k.

Someone on the forum experienced something like this because PS was using the Intel CPU for playback, but the GPU portions of the chip were disabled by the bios because there was another GPU in the laptop. Also, playback stutters and skips if not on the library tab.