With Opera 46 I have the same problem.
I also noticed that if I switch to full screen the youtube(or vimeo..) video playback becomes very smooth, just like when I minimize Opera but the video pop out remains.
So I don't think it's the hardware acceleration, maybe some 'layer' added since opera 45.

Sorry for my absence, I'm back now.
With Opera 45 portable - I disabled Hardware Acceleration on Settings and enabled Override software rendering list. Now the frames don't seem to break anymore but video is still behind audio, out of sync, in normal window and also with video pop out. So still not fixed.
But I noticed something else. If I watch youtube in a video pop out and I minimize Opera(or at least move youtube to a New window and minimize that window) the playback in the video pop out is miraculously very smooth. At least as long as youtube is minimized.
So bad playback in normal window and with the pop out, good playback in a pop out with opera or youtube window minimized. And this happens even with a 480p video

I have la latest drivers (2015) for that onboard videocard. Its a Win7 - 64 bit system with 4GB RAM.
With Opera Beta 46 or Opera 45 portable (so a new profile) and the problem is still there even at 480p. Tried a fresh Windows 7 (I had a backup). No change.
With Google Chrome Portable 59.0.3071.86 youtube plays nice even if I play multiple tabs with different videos.

Thank you, the problem remains even with a fresh install of opera 45(after deleting profile files) so
:| version 44 is for me

Hi, since Opera 45 the video playback is slow especially with youtube, and some frames seems to break. With Opera 44 the playback was smooth even with video pop out (video played in a separate window).
I have an Athlon 5000+ with an onboard nvidia video card that I use only for browsing and youtube and it worked well until Opera 45. I tried to disable the 'Battery saver', or the 'Use hardware acceleration when available' in settings but the problem remains. If I install opera 44 everything works again but it autoupdates itself very quickly.
So my question is: Can I disable something inside Opera, to go back to version 44 playback functionality, or I'll have to stay with opera 44 (by deleting 'opera_autoupdate.exe')?

Thanks.