Asus ZenFone 3 Deluxe is the first announced phone and there should be more companies announcing phones based on this faster SoC.
The Snapdragon 820 has two Kryo cores clocked at 2.15 GHz and two lower-power Kryo cores clocked at 1.59GHz while the Adreno 530 GPU was clocked at 624MHz. The Snapdragon 821 has two Kryo cores now clocked at 2.4 GHz, and two lower power Kryo cores. We didn’t get any information about the clock speeds of these two cores.
Qualcomm said that the GPU is five percent faster. In real life you should be getting up to 10 percent faster boot times and app load times could increase as much as 10 percent. This is, of course, compared to the Snapdragon 820. It looks like that the engineers managed to save some power out of the new and improved SoC as the company now claims that Snapdragon 821 delivers an incremental five percent power saving when comparing standard models.
There are a few new features including a Dual PD (PDAF), a significantly faster image auto-focus speed under a wide variety of conditions when compared to single PDAF solutions. This should essentially bring better photography on Snapdragon 821 powered phones. 
Snapdragon 821 has extended the range of laser auto focus ranging compared to Snapdragon 820. It has extended the visual focus range and improved laser focal accuracy. When it comes to the modem, the Snapdragon 821 still has the same Snapdragon X12 modem capable of Cat 12 600 Mbps download and Cat.13 150 Mbps upload speeds. Since LTE-A with speeds higher than 300 Mbps should happen at many places in 2017, Snapdragon 821 would be ready to support these fast upload and download speeds. Carrier Aggregation is the term and more and more carriers are supporting it, making the internet on your device faster. Snapdragon 820 / 821 also support LTE-U, an unlicensed spectrum that can speed things further especially in congested areas like big cities, popular squares, and sports events.
One of the most used feature on your phone is Wi-Fi. The Snapdragon 821 supports 802.11ad and 802.11ac 2x2 MU-MIMO. Multi User Multiple Input Multiple Output is a great feature which allows the Wi-Fi to be up to 2x or 3x faster than 802.11ac without MU-MIMO.
There is native support for Android 7.0 Nougat as well as Snapdragon VR SDK support including Daydream, but this is something that is also supported on Snapdragon 820 and some other chipsets from the company.
It will be interesting to see which is the next big company will relase machines based on the new Snapdragom, but it is rumored that Google 2016 Nexus devices, LeEco Le 2S, Xiaomi Mi Note 2 might end up having this faster SoC.
Snapdragon 830 is likely to be announced this year but this 10nm part is expected to show up in phones around February 2017 of Mobile World Congress 2017. Until that time Snapdragon 821 will likely be the fastest SoC to power the high end phones around the globe.
