Since you are on Windows then I highly encourage you to use this temporary proxy tool: https://forum.shotcut.org/t/shotcut-proxy-videos-auto-creation-auto-replace-tool-for-fast-smooth-editing/14519
I say temporary because 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. So while the other steps in development are taken towards performance improvement, use the above tool.