I'm having this issue as well in things like Discord. It seems in every app, including the Messages app, the keyboard hitches and stutters. It seems clear to me, as a software engineer, that there is something being processed intermittently while typing.
I was experimenting and found that it seems to be better if you disable the "Show Math Results" option in General > Keyboard. I tried disabling other options like Predictive Text and Show Predictions Inline but didn't find that made much a difference, the main one that seemed to help was disabling "Show Math Results."
I think this is meant to be a new feature but isn't working as well as they'd hoped.
Hopefully this helps other users!