Well, Linux is about choice, and that's a good thing. It may be unnecessary for you. However, for many who use Gentoo, it is by far the best OS and offers greater speed, flexibility, and features than other OS's (this is a largely subjective assessment and also varies based on an individual's needs and preferences). I can do things in Gentoo that I cannot do in any other OS, so for me, a few more minutes installing and configuring it is well worth it.
I have never claimed that Gentoo is the fastest or easiest install. It definitely is not. (Actually, Knoppix probably is... just put the livecd in and turn on the PC and you're running.) Installation time is far secondary to end functionality (we probably all agree on that) and Gentoo has far superior functionality for some of us (this is where we disagree). I don't know if you've used Gentoo. If you have and just don't really care for it, then that's fine. If you haven't, you really should use it for a while before making your decision on it. People usually have strong feelings about Gentoo... some absolutely hate it while some, like me, totally fall in love and cannot use anything else. I've tried (and continue to try) new OS's on a regular basis, including proprietary (MS, Apple) and free (many versions of Linux and BSD, other flavors of Unix, ReactOS, and others), yet I keep coming back to Gentoo.
One of the best things about free software is choice... there is no "best distro," only a best distro for a given person at a given time for a given application.