They are different processors designed to excel at different aspects of their functionality. The 810 is a stronger CPU while the 650 is a more EFFICIENT cpu. This means that the 650 will outdo the 810 during times of longer usage (like while doing a benchmark at full power) because it will produce less heat and so the phone will have to throttle the cpu much less than it does with the 810.
Yet, I do not sense a problem with throttling and all modern games play quite well on my device. The Android OS itself runs smoothly and I restart my phone once every 10 days without noticing any loss in performance during each cycle.
A ton of people are blasting z5c here and every comment I make in its defense becomes downvoted. Well, I understand that. I used to own a defective Z1 and I hate that phone. But it is quite clear that anyone with a defective device will not enjoy it, no? The z5c I own seems to be perfect (knock on wood) and I am genuinely impressed by it. There is no argument here.
I tried XC with my own hands so I am not talking out of my ass. The phone is also very fluid while using the OS and seems very fast. But the phone is also a fingerprint magnet. The phone also has extremely rounded edges. It does not feel good in my hands and I would never trade my z5c for a XC.
Why are we having this conversation? Because the Z5C is a "premium" unit, while the XC is a "mid-range" unit. People talk as if they are in the same class. The only reason XC can outperform z5c in some scenarios is just because it is using newer technology. And on that point you are right. But I STILL choose the premium experience. Because my Z5c and your XC do not have a significant difference in performance anywhere, but my z5c looks better, feels better, and can be used underwater with no ill effects (as I have done for 3 years with my z1).
The idea that XC is a clear choice is a completely wrong idea. As a seasoned power user and a tech-savvy user I still choose z5c.