I have Nexus 4 and 7, both were encrypted while on kitkat and now on lollipop, I've not seen any slow down.
Thank you -- definitely seems to be a different experience on the various devices. Even allowing for the hardware differential, it is a bit scary to see such a wide range of experiences of the same OS. Just reading a page or two here brings you all the way from "all well, no problems" to "my phone ran off with my wife" and everything in between!
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