@vegelund: Yes and keep looking at the percentage in taskmanager for a while and try to figure out what the averages are in idle mode (just article reading with no streaming going on in the background) and during some 1080p video watching or so. Rather just do it for yourself over a few days when you remember to do it, don't really have to give your answer to me, so you get an idea and you make up your own mind. But just do it if you are actually interested in it and have the time for it.
If it's 20% max in idle that is acceptable (just hope it was spiking there), but if its consistently around 40% then that's completely unacceptable and I think that would mean this is a confirmation either last few versions of Opera are broken for now or there something more to it (mining or w/e).