I too have been very frustrated with this, but it just occurred to me to try circumventing the issue with BetterTouchTool & it's working splendidly. If you don't want to pay for BTT, there is a free alternative called Jitouch which is pretty solid. I would highly recommend either/both of them for a wide range of immensely helpful uses as well as this specific issue.
In short, this is what the fix involves for either BTT or Jitouch:
(1) Make sure you have keyboard shortcuts tied to the Back and Forward functionalities in Opera settings.
(2) In BTT or Jitouch settings, map three-finger swipe gestures to their respective keyboard shortcuts. I would recommend configuring the gesture settings to be specific to the Opera app, so that it doesn't interfere with the native three-finger gesture functionality in other apps.
Not ideal, but as a workaround it's easy & effective enough to get by until Opera fixes the bug.
Confirmed, replicated the issue, found the root cause and the workaround. Atleast for me... The ball is in Opera's team to decide if its actually an issue or not.
I didnt know it can fiddle with Youtube video quality too. In case this is an issue, Opera can look into it or maybe whitelist it or create some sort of exception. In case its not an issue but rather a functionality, user should get some sort of message or one time message so he can know what Battery saver can do in nut shell...