I have noticed that when I have Fblthp in my deck, regardless of the PW, there is an incredible amount of lag at the end of my turn.
I tested this, and it is the causing me the lag:
PW: Jace, Telepath Unbound (Lvl 39)
Story Mode, Origins 1 vs Jace, Vryn’s Prodigy.
When Fibs is in my deck, it takes several seconds for the AI to take its turn. I can submit the whole deck list, if it matters. But when I take Fibs out, the pause between turns is only a second or two. Sometimes the lag starts fairly soon (perhaps because the AI searched for a card and got my Fibs), but it is always noticeable once I have my Flbthp on the field.
This is impacts events as well as the story mode.