I have a Nexus 9 and a Nexus 6, both running Android Lollipop 5.01. Almost immediately I noticed that there was a lot of lag in the UI and performance was generally slow. After doing a lot of research it appears that forced encryption in 5.0x is a huge performance drag on hardware that would otherwise be quite snappy.
Lollipop 5.1 is rumored to be available but despite being on Nexus devices I have not received OTA updates (Verizon for the 6, T-Mobile for the 9). I can sideload 5.1 on the 6 but there is not image available as of yet for the Nexus 9.
While this feature is well intentioned, the fact remains that it should never have gone into production with the enormous performance penalty it imposes. Furthermore, Google knows of the issue and has removed the forced requirement but has not yet made a fix available to those devices that already have it.
As much as I like the Nexus hardware, I would not recommend buying any Nexus device until Google disables encryption.
(Cross-posted @ Venture Chronicles)