Fans of default storage encryption will be disappointed to learn that Google has postponed mandating default encryption for Android Lollipop devices until a future version, according to a paragraph from the Android Compatibility Definition document:
9.9 Full-Disk Encryption If the device implementation has a lock screen, the device MUST support full-disk encryption of the application private data (/data partition) as well as the SD card partition if it is a permanent, non-removable part of the device. For devices supporting full-disk encryption, the full-disk encryption SHOULD be enabled all the time after the user has completed the out-of-box experience. While this requirement is stated as SHOULD for this version of the Android platform, it is very strongly RECOMMENDED as we expect this to change to MUST in the future versions of Android.
Last year, when Apple announced that iOS8 will encrypt the local storage of iPhones in a way that even the company can't decrypt, Google was quick to follow with a similar announcement for Android Lollipop. However, Google's decision seemed to be made in a hurry, and it later became clear that indeed, the decision was rushed.
Google said that all new devices running Android 5.0 will be encrypted by default, just like the Nexus 6 and the Nexus 9. Soon after these devices came out, many of their owners became frustrated that the performance wasn't as good as it should have been. It was later discovered that Android's default encryption was the culprit, which convinced many to disable the encryption.
Since then, Google seems to have updated its Android compatibility policies to say that OEMs should support storage encryption (they can't eliminate the feature from the OS), but it's up to them whether to enable it by default or not.
The main issue isn't really caused by encryption, but by the fact that devices need an AES crypto processor to encrypt the data without affecting the general performance. This is something the iPhone has had for years and why no one ever complained about its encryption performance.
Nexus 6's Snapdragon 805 came with a crypto-processor as well, but for some reason Google couldn't or wouldn't support it in Android 5.0. Because Google uses Nexus devices to define the next version of the AOSP operating system, it's possible there were some licensing issues with Qualcomm's proprietary firmware.
All ARMv8 chips from the low-end Cortex A53 to higher-end ones should support an AES crypto-processor, which should make encryption performance a non-issue for the devices being powered by these chips. However, even if we see new devices come to market with crypto-processors, it won't necessarily mean they will be encrypted by default, because Google is only making default storage encryption optional for now.
In a future version of the OS, probably Android M, Google should start mandating all OEMs to encrypt their devices by default. By then, OEMs should have plenty of warning and they should be ready to encrypt all of their devices with no impact on general system performance, thanks to ARMv8 or other custom crypto-processors.