I'm on latest ios 13 and latest FL app (Logos).
If I highlight a single word, my reading stops and waits, and waits. It's trying to do the auto-define. Eventually, it succeeds. But I have to be careful not to highlight a single word. Painful, like searching offline.
I made sure my system dictionaries are down to 1 ... New Oxford American. And I notice the issue is even for last years Noet. So, it's likely ios. Kindle has its own embedded lookup (quite snappy).
Other apps avoid the problem by not forcing an auto-lookup (eg Accordance, OliveTree, Laridian, PDF readers and so on). Is auto-lookup that needed? Vs a menu choice?