I think any KEYBerry would also be slowed down with BB Hub+ Services and the suite of BB apps.
Once those are all disabled (aside from the launcher and keyboard) it should help with performance.
With my usage, I don't really find a huge difference in performance between my OG Fold (SD855) and my KEY2...aside from gaming of course.
Do the BB apps slow the phone down this much? I've always disabled them straight away (including the launcher) and, as you point out, I find performance on the 2 and LE are quite good. Certainly good enough for everyday use.