In case you're wondering why your Nexus 6 feels so darn fast and smooth on Android 5.1, besides all the improvements on ART and possibly a lot of code cleaning and optimizations on the Framework - the device is now running full quad-core mode all the time which helps tremendously because the Kernel task scheduler can distribute the workload through all these cores so you'll get the perception that the device is not hanging anymore.
They also disabled the built-in thread migration boost routines - if you don't know what this is about it's a driver made by Qualcomm that receives a notifier from the task scheduler when one thread migrated from one cpu core to another, and to minimize perceived lag it boosted the destination core to the same, or greated, frequency than the origin core:
1 - thread moves from cpu0 to cpu2
2 - driver is notified
3 - reads the current cpu0 frequency
4 - if cpu2 current frequency is less than cpu0's read frequency it boosts cpu2 to that freq, or, if the origin freq is lower than the threshold (which is 1.7GHz by default AFAIK) it boosts to that threshold value.
5 - the boosted frequency on cpu2 stays there for at least 20ms
These migrations can occur dozens of times per-second. One of the things that I did on FK was disabling all this to conserve battery (and we don't really need all that boosting with this chip).
I'm sure Google did the math and a lot of power measurements and found out that the gains outgain the losses, so they disabled this, and I applaud them.
So these two changes balance themselves out, and I can imagine that 100% stock users will be pleasantly surprised by the improved battery life.
Unfortunately we're still dealing with full 3 second cpu boost on touch events... we don't need long of a boost, specially at 1.5GHz... Comon Google.
I don't know what other improvements they did on the Kernel, the source is not up yet, this is all I know for now.