Tried Opera on another pc → no problem
Disable "Use hardware acceleration when available" in settings → problem solved
So I guess this relates to hardware. I'm using GeForce GTX 970M and driver version 430.64.
Best posts made by cangyuyao
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Opera for WindowsRE: Lagging Pop-out Video while minimize opera windows
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Opera for WindowsRE: Lagging Pop-out Video while minimize opera windows
Same here. This also happens when the video plays in background tabs.
I think this problem appears just in recent two or three stable versions. - Opera for Windows