Now that I tested it again, yes I noticed it struggles with some 1080p and 720p videos, especially those in 60 fps. It's weird because some videos seem to work fine while others stutter and freeze all the time. Maybe it depends on which codec is used?
Browser extensions and downloads do also work by the way.
I noticed the UI is glitching out sometimes, as it shows the blue XP style title bar for a brief moment and then goes back to Win 10 style.
edit:
I installed the h264ify extension now to ensure that the h264 codec is used for every video on youtube. It definitely works so much better, even in 1080p.
Here it is running on an i5 3437U with integrated graphics.