It's interesting that you see a speed up when logged in as root. Your two explanations don't seem to be very likely. The root user is ultimately a user like anuone else.
You won't be blocked from files that are secured from regular uses, but the system still must perform these security checks (your user name is not sufficient).
It's unlikely that root's processes are given a higher priority than other users on the system, but even if this is so there's only one user logged in so it's not like you're competeing for CPU time against others. You're only competing against the daemons and kernel processes/threads.
Perhaps Apple did make debug exceptions for root, but according to others root doesn't look very well debugged at all. Icons change unexpectedly. Applications start and stop showing their ".app" extension. Maybe these are just cosmetic things, but they would definitely make me nervous :-)
Besides the debugging problems though, being logged in as root is a dangerous thing to do. You don't want to do things like unlink the kernel or remove needed files in the various hidden directories (like /dev ). Being logged in as a normal user prevents you or the programs you run from accidentally trashing these files.
I won't say that it's insane to run as root, but you should have a very good idea of what you want to do to your system that you need this unprotected account.