Sorry - I forgot to add the necessary information/specs in this post, because I did address a related issue/question here.
The current setup is:
Win 8.1 64bit, 8GB Ram, I7-4500U CPU, Shotcut-Version 19.06.15.
Current try is with H.264-Files (I am considering re-encoding to other formats as mentiones in the other post) prior editing.
Now I’ll read trough the rest of your post.