By nature, the outer and inner parts of a disc move at different speeds while a disc is spinning, regardless of format (CD, DVD, Blu-ray, HD-DVD, etc.). While DVD drives can read data at those differing speeds, Blu-ray reads at one speed. [So the spin speed has to change as the laser moves across the disc.]
That’s certainly not news, but I never knew (or didn’t remember) exactly why Blu-ray is slower than DVD for console games.
I have a hard time finding information on the web, but it looks like, ironically enough, that might be the reason why Blu-Ray has a higher capacity than HD-DVD — meaning that, if Sony had used HD-DVD in the PlayStation 3, game developers would have an easier time with the drive’s seeking speed. (But then, the PS3 wouldn’t have had HD-DVD either, since the only reason it has a blue-laser drive is precisely that Sony wanted to force Blu-ray onto the market.)