JJJ,
was using session guitarist today (actually a couple of them). No issues.

I understand that you have fast computer, but when this lag taking place, open task manager, see if anything is eating resources.

Some things I would try:

Open NI access and see if there are any updates to Kontakt and that specific Session library with issues.

Try opening Kontakt as a stand alone, and click through patterns, see if they have same lag as in DAW.

VST2 vs VST3?