This update, in not so many words, in phenomenal. In short, Google took a fast phone that performs well and made it fast-ER and perform even bett-ER.
First off, the Nexus 6 is a quad core device; however, out of the box on Android 5.0 the Nexus 6 would only utilize 2 of the 4 cores in the device for any task. This could easily be pointed out by bringing up and app such as System Monitor and view your CPU processes. Now, this device utilizes all of the cores and it is a stark difference. Recents appear instantly after a button process and overall everything just seems so much smoother as if this is the way it was meant to be to begin with.
In conclusion: The complete user experience is overhauled by being able to utilize all cores of the device, which we should’ve been able to use from the beginning. Excellent job Google, excellent job!