Factory Reset
My 2012 Nexus 7 was unusable when I updated to android lollipop 5.0 back in December. I did a factory reset and it was fixed, back to Android 4.4 speeds. After a few days of running, however, the memory leaked popped up and it needed a soft reset to get it running smoothly again. It is now running Android 5.02 and I <think> that the memory leak is fixed, as I now very rarely need to soft reset my device.
I was close to chucking the thing in the bin back in December when Android 5.0 came out. So a word a of advice; just factory reset it, and it will work ok.