It's not about hardware. My new android has twice spec than my 4 year old android phone. Yet, I'm still using my old phone for some apps. New phone has better spec, but it still has laggy scroll.
Snapdragon 810 is a big.LITTLE architecture; you don't have 8 CPUs available at the same time. Only 4.
Check your running applications, the answer to sluggishness might be there.
All 8 cores can run at the same time on the 810.
You are right, 810 has HMP. I somehow missed that.