It's a known issue that the flarum/likes extension doesn't scale well, because it loads the list of all likers for each post.
However we have yet to find out exactly how bad it is, and what the best solution is. I have personally not troubleshooted this and based on the earlier replies above, it seems like those who tried replicating didn't have a good enough data set.
The easiest solution would be to only compute the total amount of likes, save it in the database, and then show that number on the post instead of the list of people who liked. Currently the database structure and code for this doesn't exist. Then the existing relationship for likers would have to be modified to only return a subset of the likers if we want to preserve the same UI as currently. And finally a new API endpoint would have to be added to load the list of likers on demand if we want to preserve that feature when clicking "More".
The easiest short-term solution would be to create a fork of the likes extension that drops the likers and only shows the total. It would be the easiest implementation. I don't think we should drop the list of likers from the core extension, it's a nice UX of the current extension. But this could then become an option in the Likes extension so that high activity forums can hide the list of likers entirely to improve performance further.
My "likes received" extension already does the pre-calulation of like count, but only for user profiles, it doesn't change how likes are counted and returned for individual posts.
