From Matt Asay on Safari 4 at CNET:

I found page rendering to be delayed on my Mac...

That's nice. It's not about rendering. It's about Javascript. Compare on Saf 3 vs. Saf 4. There is a nice improvement.

More interesting is Asay's comment about Safari needing an API, essentially. I kinda like seeing what closed source gives us. Vive la difference, et cetera.