I think the pressing question here is a matter of time really....
One may edge out a second or so faster on a Gentoo install, but that requires you to go through hours of compiling to get that speed difference.
Arch is not compiled against any processor specific flags (only mmx, which is on all 686 class processors)... so fancy tags like sse, sse2, sss3, 3dnow, etc... are missing from Arch compiles...
So, the question again becomes: is the time spent compiling worth the slight speed boost?
I'm an Arch user, and I can say that the distribution is speedy enough, using most stock options. I've never used Gentoo for the reasons above...the >1 second increase in bootup is not worth the 8+ hours compiling on my machine.
So, just ask yourself that question... I can say that Arch is a good, solid distro to use.