While your CPU can handle the processing, you are running out of ram.
And with your video, you are forcing your computer to decide which program to use more by leaving Google Chrome open. The video clearly shows Chrome gasping for more ram as well as Shotcut. So your computer is writing to a Cache file to accommodate for not having enough memory.
I agree - before I start any SHOTCUT editing session, I reboot the computer and use SHOTCUT exclusively. Rarely have an issue now. I’ve got an i5 with 8RAM.
The other thing to check - if I have it slowing, I check that UPDATES aren’t playing games in the background with whatever they’re doing - I click “Pause Updates for 7 days” and that keeps it out of mischief. Just remember to flick it back on.