I understand the performance problems, and would like to make a simple suggestion: while Shotcut is performing some processing in the background, it would help to give some visual feedback on that. Openshot changes the cursor to a watch, and I found that very helpful.
It is always doing some processing in the background as it is heavily multi-threaded. If you are referring to jobs, the Jobs panel already shows activity. Generating waveforms and thumbnails already shows activity by their loading asynchronously and in chunks (waveform). Changing the mouse cursor should only be used when you are doing some processing in the foreground, which we try to avoid.