10%ish+ based on ART by itself. Maybe a little more since that 10% number is based on earlier builds and Google has been refining it. They claim up to 50% or something similar. For some use cases you could see a marked improvement. Android 5 also added OpenGL ES 3.1 and AEP support. Given that Qualcomm's OGL 3.1ES drivers have been borked you could see some decent improvements if Google has fixed any for Nexus.