The site found that the iPhone 6s uses a hybrid SLC/TLC NAND solution which is suited to bursty workloads. They were able to determine that the 64GB unit uses Hynix NAND but it's unclear as of yet who supplies the NAND for the 128GB variant and if other suppliers exist for the 64GB SKUs.
For those that are unfamiliar how these hybrid SLC/TLC NAND solutions work, in essence the SLC cache is made sufficiently large to avoid showing the reduced performance of TLC NAND. Any time you’re writing to the storage, the writes go to the SLC cache first before being committed to TLC NAND. As long as the overall average bandwidth demand doesn’t exceed the speed of the TLC, short-run bandwidth is solely limited by the speed of the SLC cache, which turns out to be the case for almost every normal use case.
Check out the charts below to see how the iPhone 6s compares to the competition and hit the link for more details...