I've seen the poleaxe Byrdi or NuberT use have an OP insta stab. It's almost as fast as a 1 hander, considering their "normal" speed is much less.
Eh are you sure you ain't talking about stun? Polearm stab is fast and when I am positioned good to an enemy after I hit him I can get a free stab. But that depends a lot on the opponents skill/ping/idk. Won't usually work on people who know how I fight, like GTX, who will downblock after he got hit. Just downblock and see if you get "instastabed".
Also when fighting close to each other the stab of a poleaxe for example will ofc connect instantly after beeing released, as the tip is already touching your body. But same goes for swings. Personally I liked combat more, when these early attacks were more likely to bounce off, but thats like 18 months ago