Simple Shape Mask tanking performance, lagging, and crashing?

This could likely be a cause of the crashes. That turns on additional multi-threading–as explained in the FAQ (link at the top of this page and in web menu) and documentation. That can also be the source of lag since it ensures every video frame is shown regardless of how long it takes and “lag” = abnormal latency from things responding slowly.

Wouldve been nice for you to mention that the log is stored at:

It is in the FAQ. Also, the very first link shared with you explains app data directory contains the log and shows how to get to the folder in the Shotcut menu.

Or, just make sure you use File > Backup and Save when running a different version if you actually feel the need to save. Autosave in Shotcut does not overwrite your current project file on some timer. Rather, in the background, it periodically saves to a file in a special folder in app data.