It's not just the case that it's debugging code, it's that the code hasn't been OPTIMIZED yet

Well, inside Apple it has, but not for the public yet.
You can expect to see performance gains when the final version is released. And that person talking about single user mode obviously has no idea what he/she's talking about - single user mode is for DEBUGGING YOUR SYSTEM when it gets farked, something that's been rolled over since BSD most likely. Like debugging your configuration files and things.
