Why does the HTML5 YouTube trial work so poorly in Safari?

Surely, I would expect that the HTML5 trial for YouTube would work better in Safari than Flash — something Apple doesn't like or promote.

But on my SSD, top-of-the-line MacBook Pro (best graphics card and processor), it always crashes, freezes, and is a pain.

Does anyone know why? Should I install more RAM? (I have 4GB.)

I understand that it's an experimental trial, but it's very strange.

This happened on my old MacBook Pro (also very powerful) and even after I re-set Safari or re-installed OS X on my current one.