I noticed the same (i7-920, FF 3.6) - after removing the fixed position bar (using firebug) the site at least seems significantly less sluggish. Weird how a seemingly simple thing can hurt performance so much (or maybe it's just a psychological effect ?)

On a large screen I find it visually more pleasing than the last version but I stoppped using the site completely after they introduced that silly ajax-abuse and the new one wouldn't make me come back if for that reason alone.