My tests had me arrive at a different conclusion. For me Parallel Processing is only an issue when a specific kind of video is used and the Convert To Friendly option is not used on it. For me that’s videos from youtube.
Here is a video demo I did using the Bruno Mars music video “Leave The Door Open” directly from the 4K youtube upload that we were using in the last thread for Time Remap here. The first clip and second clip are exactly the same in terms of the section that it is slowed down and both exports used Parallel Processing. But only the second video shows this flash at exactly 29sec19ms:
The difference is that the first export in the demo used a converted to edit friendly sub-clip whereas the second did not convert a sub-clip so it’s the youtube video as is allowed by Shotcut to use Time Remap on and it’s on that one that the above flash appears. Not in the demo but I can confirm is that if I turn Parallel Processing off then the second export without the converted to edit friendly sub-clip does not produce a flash in the clip.