Turn off the battery saver at the URL opera://settings/batterySaver and disable any limiters you have enabled in the GX Control sidebar panel. Then, restart Opera to see if there's any difference.
If no go, try disabling your mods and extensions one by one at the URLs opera://mods and opera://extensions and test to see if one is causing it.
Besides that, see https://reddit.com/r/operabrowser/comments/1d1vjal/youtube_issues_ads_no_sound_skipping_performance/ if you're using Opera's adblocker.