Suggest for you. Always drag a video clip to the timeline first. You can always move it where ever you want once it's there. You can even add a title in front of it, and the timeline settings will not change.  

My last gasp.... you tried using custom export settings?

Instead of using keyframes since your are slowing down the movie at  a constant speed, why not just type in the speed? Try 50 and see what happens... 

Obvious question.... timeline settings? I have no 720P footage to test.

Wow... that is a horrible story. I don't have anything synced to anything on my computer or cellphone.

Well, to get ride of the bars, he will have to zoom into the video with 2D-3D effects. Choose your poison... You can crop the video to ''remove'' the black bars on the side, but when played back on a wide screen, the bars will be there. 

And to turn it off..... you have to go to Settings>Advance

You say you created a video and imported it into PS23? Questions: 1. So this video was not made in PS23, correct? 2. What screen resolution was the video exported in and then put into PS23? 3. Or are you starting from ''scratch'', which would be the easiest way to do this.   

What's the cause? You have a variable frame rate video. Video editors are constant (fixed) frame rate editing tools.  What MediaInfo shows you is just that: Frame rate mode                          : Variable Frame rate                ...

Hmmm... see my computer specs below. Before this build below, it was a Ryzen 7 1700X OC and water cooled with 16GB RAM. Never a problem with that computer editing software. I have zero problems playing back UHD 4K. None of my exports (using video captured with Panasonic cameras), including 50P 1080, 25P 4K, 50P 4K were ever out of ...