Try this first, go to settings, apps, in the area where it says your apps, tap on the 3 line on the right hand side, toggle on show system apps, in the search box, type in Keyboard, you will see Samsung Keyboard, open that, go to storage, and clear cache. If it is still laggy. Then you can , try to reset to default. Go to settings, type in the search box Keyboard, open it, scroll down to reset settings.
Keyboard reside in General Management of settings. Quicker using search.
