Extreme lag (in Preview) to the point of unusability in Shotcut

[DRM]

First, download the latest version of Shotcut which is 20.11.28 that was released last month.
Second, have you used Proxy along with Preview Scaling?

Proxy Editing

Introduction Proxy editing is the process of creating and using low resolution videos and images in places of the original or optimized (Convert to Edit-friendly) files. This provides the advantage that your computer has less work to do in realtime while editing: decoding, scaling, and effects. This is, quite simply, because there are less pixels to compute. Then, when you are ready to export, it will use the original (or optimized) files for full quality. Performance is optimal when the pre…