I believe this is caused by the Intel HD Graphics manager being in enhanced power save mode, it seems as if the gamma of the scene changes when the overlay is being drawn which causes more gpu utilization. You can run opera with a dedicated gpu to bypass this, but I cant tell if disabling Max Battery Life and Enhanced Power Save Mode work, as it changes the brightness too. I would say it works. This would be nice to have fixed though, as it affects all 6th,7th,8th, and possibly 9th generation Intel mobile graphics adapters.
Edit: after more testing, my conclusion may not be correct at all, the same still occurs if you run opera with a specific gpu, because it defaults each process created afterwards to the integrated chip no matter what you do. I do think its related to the Intel driver though, and it seems like the gamma changes, thats all I know for sure.