There's a massive thread about this... the disc versions seem to be unplayable on the 4GB Xbox Slims, although gamers with other Xboxes have had issues as well.
And it has nothing to to with the speed that data can be read from the disc. The Xbox drives can read between 8 Mbps and 10 Mbps.
This is why super-compressed games like Mass Effect 3 can stream content rather effectively with no lag even though they're not installed to the local drive.
Maybe Telltale didn't do a good job optimizing the game for disc release, but the issue is with the coding and not with the hardware.