Android 5.0 Lollipop introduces full device encryption, and for those who are receiving the update by OTA, it won’t be enabled by default. However, devices shipped with Android Lollipop, such as the Nexus 6, will have the encryption turned on by default. In benchmarks done by AnandTech, it seems that full device encryption comes at the price of performance.
In the benchmarks done, an encrypted Nexus 6 performs much worse than a Nexus 6 with encryption turned off. The performance hit is so large, that even a Nexus 5 updated to Lollipop performs better than the encrypted Nexus 6.
To make matters worse, a normal user will not be able to turn off full device encryption. In order to do so, some knowledge of flashing an Android device is necessary, which isn’t something that everyone can do easily. If you’re keen to try, here’s a guide to turn full device encryption off on the Nexus 6. On a side note, battery life might suffer as well from the increased workload of encrypting and decrypting.
Perhaps the implementation of UFS 2.0 in future smartphones will solve this issue by offering a huge boost in storage speed. But for now, Google would have to either enable full device encryption to be turned off easily by normal users, or implement it better so that it wouldn’t cause too much of a performance degradation.