I am also experience extreme lag on the s6 not just in the keyboard but in the whole system. As you say it gets unbearable and happens quite often. It's not just stuttering in the launcher, but in apps, setting, everything. It is often accompanied by the phone getting very hot.
I unusually fix this by booting into the boot loader menu, clearing the cache and then rebooting. It fixes things for a little while.
I am hoping this is a result of the memory management bug that has been discussed previously on this forum and that a future update will fix things. If not I will have to look for a new phone.
Sent from my SAMSUNG-SM-G925A using Tapatalk