There are certainly issues with Shotcut’s playback performance but it’s never been as bad for me as what you are describing. I think your PC is more of an issue than you realize.
What version number of Shocut are you using?
It was announced that this year is supposed to be the year when Shotcut’s performance is updated and improved. The first step was taken in the last released version 20.02.17 by debuting the Preview Scaling feature which does help with playback performance in regards to effects being applied. In case you are not aware here is the page to read about it and where to download: https://shotcut.org/blog/new-release-200217/
While the other steps in development are taken towards performance improvement, if you are on Windows then I highly encourage you to also use this proxy tool that was shared by @KKnBB in the meantime: https://forum.shotcut.org/t/shotcut-proxy-videos-auto-creation-auto-replace-tool-for-fast-smooth-editing/14519