"Apparently, Google has not merged the various drivers that optimize Qualcomm's QCE module for encryption and decryption into AOSP. The generally-assumed reason

"Apparently, Google has not merged the various drivers that optimize Qualcomm's QCE module for encryption and decryption into AOSP. The generally-assumed reason is that this code is proprietary. Without these optimizations, the Nexus 6's hardware decryption module on the Snapdragon 805 is essentially hamstrung." http://www.androidpolice.com/2014/11/20/anandtech-posts-side-by-side-nand-performance-for-nexus-6-encrypted-vs-unencrypted-its-not-pretty/ http://www.androidpolice.com/2014/11/20/anandtech-posts-side... I believe the 5.1 update fixed the QCE issue.