When you open emoji hangs loading and console contains an error Type error.

TypeError: undefined is not an object (evaluating 'emoji[category].length')

 

Try running the support tool in the AdminCP to clear all of your caches. If you continue to experience trouble, please submit a ticket.

Does this happen with native or twitter emojis because on all my sites twitter emojis take long to load while native are pretty much instant. I prefer twitter, just takes too long.

Slow loading Twitter emoji would be different from a javascript error.

(I kind of suspect he has a custom emoji stored that is orphaned and not in a category, but that's just a guess)