The map design process and event creation will be what takes them the longest, because most assets/textures will already be available to them, and bosses are typically just enlarged versions of regular enemies. That's just a simple break down, and I'm very aware that there is more to it, but not that much more and it really shouldn't take that long, at least not with good hardware and software. However, if rumors are true, Bungie's hardware and software are way below par. EDIT: And by rumor, I mean Bungie themselves have complained about it, but you never know if it is just an excuse.

Oohhhh now that's the 100million dollars question!