What is your version of Shotcut? In the recent versions, there are 3 Crop filters. In version 20.02 there is a known bug when using Crop:Rectangle or Crop:Circle in conjunction with Preview Scaling. Turn off Preview Scaling in Settings. This is fixed for the next version.
That is the normal and expect performance of Shotcut with HD material on a dual core system. Export to have the video play smoothy.
Actually, this was due to a signing entitlements problem for Apple’s new requirements for notarization. It was fixed a couple of weeks ago, and new build uploaded. You can fix this by re-downloading and installing version 20.02.17