One of the headline features of the New Xbox Experience is the ability to load games onto the 360's hard-drive rather than run them from the disc. That's intended to speed up load times and avoid frustrating drive noise. However it seems that not every title stands to benefit from the system in the same way: in Halo 3, for instance, loading from the hard-drive actually makes things slower.
The problem arises because of the way Halo 3's developers tried to pre-empt loading time frustrations. The game automatically copies map data onto the Xbox 360's hard-drive; now, when you try to install the whole title to the disc using the NXE system, Halo 3 is actually copying from one part of the drive to another. That takes longer than it does to go from disc to drive.
Developers Bungie have explained that Halo 3 was shipped before Microsoft finalized the NXE disc-copy feature, and as such they were unable to make the game compatible. While a patch is possible they don't sound especially keen, describing it as "a significant undertaking" especially when gamers aren't actually prevented from playing Halo 3. Perhaps all that's needed is a warning that flashes up on the Xbox itself should gamers try to copy the title across, mentioning that performance for certain games won't be improved.