Before cleaning your WordPress, make sure you have a backup for it. I recommend you UpdraftPlus if you don’t have any yet.
Use the plugin WP Maintenance Mode to enable the maintenance mode. This will serve a static page to everyone, except administrators.
Run the plugin WP Clean Up and delete all the drafts – Check if there is any draft that is not an RSS imported post (categorized as ‘imported-photo’).
Clean Media Cleaner
Run the plugin Media Cleaner to locate and erase the orphan files, broken media, and files you do not need.
Disable Maintenance Mode
Disable the maintenance mode.
Test your WordPress install
Test everything in incognito mode or in a new browser (so nothing is cached). Double test everything with another computer.