Obviously if you have node doing less it's going to be "faster". Or is it? I'm not really sure that offloading tasks qualifies as being faster. I'd like to see some data on this though. Basic caching as supplied with connect should serve static files pretty damn fast I would think. Perhaps nginx does that for a living and kicks ass, but node shouldn't be all that bad.
-Node has more cycles to compute things?
-The server node's on has less contention to deal with (disk, network)?
-Putting statics on another server allows the browser more simultaneous connections when loading the page, so it loads faster by default?
"Don't use Node.js for static assets" is an interesting observation but I'd like to know exactly why.
The approach I favour is to put varnish in front of a classic slow script webserver (tornado, but use node if you prefer).
Tornado has this neat way of baking-in versioning to static content so clients (and proxies) can do aggressive caching http://www.tornadoweb.org/documentation/overview.html#static...
Best of both worlds.