I have just finished and uploaded a 10 min video to YT.
The video is edited in a much more complex editor, but to have a good
quality upload, I rendered the final video in PD's .wmv full HD.
I had some "left-overs", clips I wished to make a short, much faster video
from- doing the entire edit in PD9.
The left-overs I wanted to use was mainly .mp4's, 1280x720 / 59,94. Some
straight off camera, and some lightly tampered with (in PD).

Started with background, sky- and clouds-clip timelapsed (in rendered .mp4)- 22 sec.
Wanted to make a tiny colour- change during the clip. (see img 1)
What then was to be seen in the preview window was too awful to proceed with. It
would not be possible to see what really was going on.
As more or less a test, I produced a new 22 sec file .mp4 of the clip with the keyframe
adjustments.. That made a difference toreable to go on.
(workaround,workaround, workaround.....)

I then imported a couple of other 20-22 sec clips, now wanting to set up motion for
them in Modify. From experience, I know that I probably will have to go back and forth
between Modify window and timeline several times to have my keyframing stick.
There will fewer clicks if I use .png's than some seconds of a video clip.
With the .mp4's I had at hand today, it seemed impossible, just impossible to edit
motion. I kept trying with 2 (18-20 sec clips) for 40 min before I gave up. It simply just
would not work.

Img. 2

Firstly, I find it very annoying that a clip changes size without my approval in the
form of a keyframe.
In photo 1, you see my clips, none having any effect attached. None having audio.
In photo 2, I am placing the second clip where I want it to start.
Notice, I havn't set any keyframe yet.
In photo 3, I'm moving the slider to a point where I am thinking about setting a keyframe.
But,before I, the editor, makes that decision, PD has applied a motion and resized the clip.

In photo 4 I think I have set the keyframes I want. However - when I play the
motion in modify window.....it changes my settings! Again!
And holy smokes....when going back to the preview window....look at it.
PD, within itself even, can't decide what to make of it!
And mind you, these clips were produced in PD9 beforehand, but not today.

Since I've just finished editing a video with many pre-renders, complex keyframing and
loads of overlays, it's no use talking about computerpower, graphic card or free space on
C: drive or preview settings. It just doesn't work.
Might it work with .wmv's, mpeg2's, m2t's......?
Well.....mp4 is a very common format these days, isn't it?

File info from the net:

AC filMovie or video clip that uses MPEG-4 compression, a standard developed by the Moving Picture Experts Group (MPEG); commonly used for sharing video files on the Internet.
The MPEG-4 video format uses separate compression for audio and video tracks. Video is compressed with MPEG-4 video encoding. Audio is compressed using AAC compression, the same type of audio compression used in .Aes.

Just something.
https://www.petitpoisvideo.com