I have had the exact same issues. Not number 3.
It all started after updating my Windows 10 september 16. Today I figured I might try to uninstall some of the updates.
Three updates were installed that day.
KB5030211, KB5029919 and Servicingstack 10.0.19041.3385
I first paused any updates for the next 9 days.
Then I uninstalled KB5030211. Rebooted and tried to open my project in Shotcut. No difference. Slow and sluggish.
Then I uninstalled KB5029919. Rebooted and fired up Shotcut. Everything was back to normal.
Give it a try Good luck, hope it helps.
Sorry to hear about your issues. Can you please tell us:
The version of shotcut you are using
Some basic computer specs (RAM, CPU etc)
You mention preview scaling, but do you also have proxy editing enabled? That is extremely important, and without it preview scaling won’t help as much as you might think (especially if your system is struggling).
Other information that will help potentially diagnose your problem:
Did this happen after a recent upgrade, or was the version you are using working just fine, and then one day all hell broke loose for no apparent reason? If this happened after an upgrade, try reverting to an older version that you know worked.
Have you tried a complete uninstall/reinstall, including removing registry entries during the reinstall process? Full instructions on how to do that below.
I would start there. Then begin a new project and import a short video that is NOT in your current project (the one that’s now causing you problems). Edit that and see how it behaves.
This could also be related to certain add-ons like grammarly or even your antivirus.
Do a clean windows boot with startup items disabled (assuming windows) and test again.
If you still have issues and you can’t narrow it down to a windows update (as per @Truls post above) then you could try installing on a different (preferably new) user profile, assuming once again the OS is windows.
OK so there’s a clue. Since you’re on a macbook, my windows specific suggestions and the windows updates aren’t relevant. Definitely try reverting to a previous version, and enable proxy when working with 1080p or 4k video.