[heinzgruber]

so there is no chance to get 1080p working on Pi3?

That is out of our hands really.
libwidevine contains a software decoder (as well as decryption) and is closed source so isn't something we can optimise.
It doesn't appear to use multiple cores as well as ffmpeg (normally two cores seem relatively idle).

Overclocking may help, but I don't expect you will get far enough for 1080p software decode.

okay. thanks. 'm already at 1350 MHz and it still stutter.