Seriously, if you have modern hardware the puny human mind isn't going to comprehend the difference in speed between one well-maintained distro and another.
More customisability is what I get from Gentoo over Arch. I used Arch for awhile and it is a good distro, but I have this perverse need to decide for myself what dependencies to build everything against. I used to do it with LFS and CLFS, but that started taking too much of my time hunting down and fixing build errors. With Gentoo, I get the micro-management I like and someone else gets to create all the patches.
The initial Gentoo installation can take a little while especially if you plan to install X and a DE. After that, you're not installing that much, just package upgrades. Those can build and install in the background while you continue to work. Of course, I have six machines and use distcc so that may be why things build a little faster for me