I've never seen Youtube not drop frames. Best it seems to be able to do is about 1 dropped frame per second, regardless of OS and browser. It also seems to almost always use codecs which require CPU decoding, which is obviously terrible for power efficiency (though it might save them a little bit in bandwidth I guess).

I don't watch long videos on YT for this reason, I download them and use a real video player, which can play video without dropping frames at 2 % CPU. This also gives me my mandatory archival copy, as interesting things are often deleted or channels deleted / terminated.