Qualcomm was looking to put the disastrous Snapdragon 810 in the rear view mirror when it began shipping the Snapdragon 820 a while back. Now, it's putting more distance between itself and ARM's reference cores with the Snapdragon 821. This is the second chip with Qualcomm's custom 64-bit CPU cores, and it's apparently as much as 10% faster than the 820.
Like the Snapdragon 820, the 821 is a quad-core chip based on the custom Kryo cores. The 810 was octa-core, based on the ARM reference Cortex-A57 and A53. With the 820 (and now 821), the four Kryo cores operating at varying clock speeds can offer better performance without as much heat. The Snapdragon 821 will come in clock speeds as high as 2.4GHz compared to the 2.15GHz for the 820. Maybe that's the entire basis of Qualcomm's 10% improvement claim.
This isn't the end of the road for the 820. We'll probably still see plenty of phones shipping with it before the Snapdragon 821 becomes more common. Maybe those Nexus phones will be among the first with the 821?