I was checking the messages that I typed on my phone recently. The most I typed in a message recently was only 120 words or maybe about 700 characters. I use Thumb Keyboard with no auto-correct or prediction of any kind. I did not experience any keyboard lag, but this probably didn't reach the threshold where you notice any slow down.
Sorry, but I'm not about to write a long message just for the sake of testing this out. I'll keep this in mind in case I do type a long message in the future.