Why is this fast?

It's all quite simple. It develops flow over the main. If the mast is upright or raked back the rig can only develop stalled 'drag' forces, but by raking the mast forward, the air can escape upwards and thus develop flow across the sail. Bethwaite talks about 'roll over vortexes' that work their way up the lee side of a poled out jib, drawing air across it and helping to develop further flow across the main. Personally we have found that when you get the mast well forward and hoist the jib up, there is enough slack in the forestay to start making the jib behave a bit like a  spinnaker. Either way it is waaay faster to have the rig forward than not.
If you look at the fundamentals of a Star rig, you can appreciate how important this is. For a start the chainplates for the upper shrouds are ahead of the mast, so that as the rig goes forward, the shrounds loosen and the bend comes out of the mast. Also the mast heel is free to move in Derlin or rollers, so that once the mast has hit the front end of the mast gate, the heel can slide back 1"-2" for a bit more forward rake. Thirdly the spreaders are free to swing forwards, so that the leeward shorunds and spreaders do not cut into the main and spoil the sail shape.
It is generally best to have the rig max forward in the light [helps keep the boom out too], so that the mast tip is almost over the bow, with a slight inversion in the mast to develop am even better flow and the crew lying on the deck as far forward as he can go [hanging onto the forstay with one hand], and enough heel to keep the tiller neutral. As the breeze comes on everything starts coming back not only to keep the mast in the boat, but to make it controlable [think laser with no kicker downwind]. Have a look at http://www.quantumsails.com/products/onedesign/cni.asp?NID=1 1&CID=5 Mark Reynolds gen on it.
Also have a look at http://www.stargear.net/articles/torbenjibe.html for how the top guys gybe with the rig forward and the crew on the foredeck.
As for high performance boats - i'll bet there's not a lot quicker than the Star in the light stuff dead downwind, and until the apparent comes up enough for a boat to plane the fundamentals still apply. Stars plane too you know! Yes - the masts are fragile, but that is the nature of the boat. It's pretty easy to sail with the rig back and the mast will be fairly safe[ excluding gear breakages etc]. But let it forward if you want to win. Likewise it would be dead easy to make a bigger heavier section with fixed spreaders that was really damage tolerant, but like all racing equipment you have to push the design limits to go faster and if you have ever seen a Star mast in the flesh you'll know just how much of a noodle it is. Keeping the rig in the boat is all part of the sailing stars. It might seem an odd boat to much of the UK but the worlds best sailors along with around 3000 active boats worldwide can't all be wrong! Bear in mind no-one has retained a Star Worlds in over 20 years, which does say something about the top level in the class.

Kenny - Star 7665

For heaven's sake Kenny, if people who know what they are talking about butt in, it will ruin the whole forum.

Seriously, very interesting, thanks.