I was experiencing some weirdness earlier today on my 6 Plus. A text wouldn't sent and FB wouldn't update! But, a reboot fixing everything with iPhones so....I'm good now
Just try this simple test :-
Backup your iphone using itunes.
Delete the third party keyboard app from your iphone and try texting using the stock keyboard.
If it does not hang, then there is surely a problem with the third party keyboard app.