Because we're not going to change the interface to make the internals faster. And we might as well spend the time that the RC is out there looking for bugs to make things faster as well.
It's not a release candidate if you literally state that it is not a candidate for release.
If I install the final version I'd expect that means it's the exact same version as the most recent release candidate and no critical bugs were found during that period. Not the most recent release candidate, plus a bunch of tweaks that were made while everyone was testing. And if you're already assuming there will be another release candidate before a final, then this isn't a release candidate.