FCP sluggish; what files and folders to flush to work better

Please post an EtreCheck report https://etrecheck.com giving the application Full Disk Access in Privacy & Security Sysyytem Settings. Copy the report and use the Add Text button to paste the result into reply box. 

Verify the PERCENTAGE of free space on your system drive.

I highly recommend the app Arctic to manage FCP Libraries. It lets you track and manage render files, analysis files, offline content, etc, very easily.