One of the features in Android 5.0 Lollipop is encryption, which encrypts all your data and can only be decrypted when you unlock the phone using the passcode, making it unusable to someone without the passcode even if they can access the memory.
This feature is disabled by default on devices that got updated to Lollipop (such as the Nexus 5) and has to be enabled separately but the Nexus 6 and the Nexus 9 come with it enabled by default (which cannot be disabled). Unfortunately, that may not be the best idea, as AnandTech’s tests have shown, the encryption feature has a severe performance penalty.
AnandTech ran a couple of benchmarks on the standard Nexus 6 and a special version sent by Motorola with the encryption disabled. In each of the tests, the Nexus 6 with encryption disabled was significantly faster than the standard version. They repeated the test with the Nexus 5 (which has a switch in the settings to enable encryption) and even with that device, the performance was greatly reduced after enabling the feature.
The jury is still out on the actual benefits of this feature, since it relies on the user enabling passcode lock, without which it is useless. The performance penalty, however, is not optional and you get it as long as the feature remains on, which can’t be disabled on some devices. While the intentions behind it may be noble, Google needs to rethink the execution here.