Welcome to Opera's Forums @Momohirai!

The HW acceleration option is from settings but there is another option into Flags about overwrite blacklisted GPUs.
Did you tested both?

I had Flash Player flickering problems using the NVDA GPu whereas using integrated Intel HD3000 works flawlessly so
I've decided to uninstall Flash Player and work only with HTML5, (I just have an Opera Portable with Flash if needed for some reason).

BTW, Youtube usually works as HTML5 if I'm not wrong.
Try without extensions or with a clean profile, check the How to on the Opera Windows Forum FAQ.