tom23 clicking on the "Fixup" button at FreeFlarum settings will do 3 things:
- It clears Flarum cache (this is the same as going to Admin > Dashboard > Tools > Clear cache);
- It migrates the forum database;
- It synchronizes the forum with our master forum (if there are new files for master forum for example, then they will be copied/linked over to other forums);
Cache generally exists in order to make the performance and loading times better, so I admit that I am not sure how would this affect the performance, to be honest. As for running this automatically, this is already done during extension/Flarum updates, but otherwise you have to run it manually. This is because running all these tasks drains the server resources, so if it would be ran frequently, it would actually slow down forums