I have an old lightweight laptop that I use for youtube videos from time to time.
Page loads take 3-10 seconds. Video decoding, once it gets going, is great due to dedicated MPEG hardware. But the site itself - well, old man gets there eventually.
Edit: already stripped overhead to the bone by running Bodhi Linux. I may try FF over Chromium in case there's more performance to be had. But the 2GB RAM footprint is really pushing it these days.