Thanks for your report and comments. There isn’t really enough information in your report for us to act on. It would be helpful for you to tell us what version of Shotcut you are using.
The next time you experience a crash, it would be good if you could send us the Shorcut log file. Here are some instructions that can help you find it. Be sure to get the file before re-starting Shotcut or the file will be over written and any crash information will be lost.
Also, for lagging, here are some features that might help you: