The Nexus 6 had a lot of fine qualities, but the sluggish storage performance was a disappointment. This was mostly due to the automatic device encryption, which was managed by software rather than hardware. In today's Reddit AMA, the Nexus team was asked about encryption support in the Nexus 5X and 6P. VP of Engineering Dave Burke responded, saying it's still software-based, but it should be even faster than hardware encryption this time.
The big difference in this year's Nexus phones is that they're running the Snapdragon 810 and 808, which are 64-bit SoCs with the ARMv8 instruction set. According to Burke, there are cryptography extensions provided by ARMv8 that offer better performance than the AES hardware encryption you could do on the Qualcomm SoC.
DB> Encryption is software accelerated. Specifically the ARMv8 as part of 64-bit support has a number of instructions that provides better performance than the AES hardware options on the SoC.
Based on this, there shouldn't be a compelling reason to go around flashing modified boot images so you can disable encryption on the new Nexus phones. They're still running on eMMC 5.0 rather than the faster UFS storage Samsung has started producing, so it's not like it's going to be screaming fast in the storage department anyway.