Thanks, friends.
I use my iphone X to take photos and videos and imovie to edit. It's very basic so I think I may try to use a better app on my laptop. it's an HP intel i5 machine in Windows 7, not fast of course. The VP version is 8.45. Surely there will be a lot to learn.
Understand longer image will have smoother transition. https://wetransfer.com/downloads/4a0c3ac7b648194de157f4f76b64219720200923124502/ab1fcb are the two test videos I made, one by imovie and one by VP. Size of Imovie.mov is smaller than NCH.mov but apparently smoother. That may be size of video different. I changed settings in VP with higher fps and resolution and the output file is 5-8 time large in size and took ages to compile (maybe my laptop is slow).
Anyway thanks for your feedback.
Thomas