This is strange. Is there any confirmation from the dev team or anyone in the know that wse2 actually causes negative performance impacts, and why? From the amazingly positive experience I've had with it, it's hard to imagine. I feel like it must be some kind of settings issue on the player's part not being tweaked appropriately or something along those lines. For starters, have you tried setting the fps limit to 60 for example? Had you applied any third party tweaks to the warband .exe prior to wse, like graphics card control panel type stuff, to make it run smoother? If so, you might have to reapply to the wse2 .exe or whatever the new one it runs on is.
I'dunno, there's probably a bunch of stuff to consider, but just based on the huge performance boost it gave me, I don't understand why it would have the opposite effect on anyone else. Unless there are legitimate incompatibility issues with certain machines or operating systems or whatnot, which I've not seen confirmation on. Not that I've been looking. In any case, good luck with that to those having issues, hope you get it sorted so you can enjoy the benefits of wse2 like the rest of us.