The feature will use a background process that will run from a cron event and can take a while to run the feature depending on how much CSS needs to be removed on the site. The feature will use a custom database table to store the data in the named
If you have the remove unused CSS feature enabled you should check to see how large the database table using phpMyAdmin is. The expected range for normal usage would be 20-50 MB in size. If the database table is in the 200-450MB range then the recommendation would be to disable the beta feature in WP Rocket in wp-admin;
WP Rocket > File Optimization > Remove Unused CSS
Then using phpMyAdmin or a similar database access application to empty the custom database table. By emptying the database table it will still keep the database table would remove the records that are stored in it. Removing unused CSS on your site is still a valid performance recommend but I would wait till more of the beta issues have been resolved in the feature provided by WP Rocket and it is production-ready. If you are set on using the core vitals feature in the WP Rocket plugin I would recommend first testing it out on a staging site.