Are you performing the proper WSUS maintenance including but not limited to running the Server Cleanup Wizard (SCW), declining superseded updates, running the SQL Indexing script, etc.?
https://www.ajtek.ca/wsus/how-to-setup-manage-and-maintain-wsus-part-8-wsus-server-maintenance/
To quote myself:
Just because you’ve installed a new WSUS server, doesn’t mean that it’s clean or optimized; it just means that it’s NEW!
Also, if you just reinstalled WSUS, and you're experiencing prior sync history, then it sounds like you didn't reinstall it correctly.
https://www.ajtek.ca/wsus/how-to-remove-wsus-completely-and-reinstall-it/