Hi,
I recommend you create a test MP4 video from the Sample Project. It's included with PTE AV Studio. Browse the main menu > Help > Open Sample Project.
Create a MP4 video with the following video options: "1920 x 1080, "High quality", "60p". Keep Motion Blur DISABLED.
First video create with unticked "Hardware acceleration" option. The second file create with ticked option "Hardware...".
Check the file size, it should be about 150 - 300 MB.
Then open each video file in QuickTime player, compare both videos. And compare with a full screen preview for Sample Project in PTE AV Studio.
It will help us determine where the problem with smoothness occurs.