While the majority of Nexus 6 owners are still waiting for the now official Android 5.1 Lollipop to start seeding, power users and developers have already had the chance to not just load the new firmware image on their own units manually, but to even dig a little deeper in order to make sense of the changes Google included.
According to Francisco Franco, the developer behind the extremely popular custom Android franco.kernel, the changes are not at all trivial. Writing on his Google+ page, Francisco addresses speculation that has the Nexus 6 feeling even smoother under Lollipop 5.1. According to the dev, this is no placebo, and people are right.
Apparently, apart from improvements made to the ART runtime and any other possible optimizations of code, the new firmware also forces the Nexus 6's quad-core Qualcomm Snapdragon 805 processor to run all four cores at all times:
"... 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."
This is where your performance gain comes from. As for the part about better efficiency, we're told that Google has disabled something called 'built-in thread migration routines' that helps your battery survive for longer. Put simply, the feature basically kicks in whenever one task has to be migrated to a different from its current CPU core. In those instances, the task scheduler would before tell the destination core to match or exceed the current frequency of the origin core, lowering perceived lag. Since these migrations can apparently occur as often as every 20 ms, disabling them equals savings.
A good time to be a Nexus 6 user, no?