The 8X features Qualcomm's dual-core Krait Snapdragon S4 (MSM8960) running at 1.5GHz. The 8S on the other hand uses a lower clocked MSM8627 (1GHz) with a slower Adreno 305 GPU
Just that part seems like a big deal. I bet if you used both side by side, just the CPU difference would be obvious, let alone the GPU difference.
The two phones differ in the amount of on-package DRAM (1GB for the 8X and 512MB for the 8S)
Add that in, and you're going to see a large difference in UX. Load a few programs, and the 8S is going to be bogging down while the 8X is still cruising along.
Storage is split between the two as well, with the 8X featuring 16GB of NAND on-board (no microSD card slot), and the 8S with only 4GB integrated and a microSD card slot for expansion.
That's a big difference... though I think it curious that the lower priced phone has room for expansion while the pricier one does not. Hmm...
But on to the issue at hand, to me, personally, there's an obvious difference between the two... and more than the generational gap that 8MP vs 10MP represents- though that's pretty big to actual photographers- my wife had to school me on that by blowing up two pictures from said cameras. The difference is night and day, even on that!