The last speed test was between PS3 Slim running 2.76 and PS3 Fat running 2.80. That was an unfair comparison. The loading time difference on Batman Arkham Asylum between the slim and the fat was due to the slim not playing the intro clip that displays "Playstation 3" on the screen. My fat, running 3.00, does not display that screen when you start a game, meaning it has identical loading time to a slim for games.
The fat was 7 seconds faster loading a blu-ray movie.
So far, the slim has proven to be equal or slower in everything, but I'm not complaining since the system has a lot lower power consumption and price. I do, however, thoroughly enjoy playing emulated games on my Linux partition using my Dualshock3 thru bluetooth - something a slim can never do.