Technically CentOS is not the slowest, as Whitebox takes the wooden spoon but you get the general idea. :)
I feel that the CentOS team does an outstanding job. I have never used any other RHEL rebuilds, because CentOS is the clear leader in RHEL rebuilds. Nothing is more important than quality, and no one does it better than CentOS. In many ways, CentOS has a harder job than Red Hat does. Regardless of how the CentOS engineers might choose to solve a problem, they are committed to following in Red Hat's footsteps, no matter how difficult it is. CentOS also adds value on top of RHEL by providing the centosplus software, and by supporting architectures that RHEL doesn't. I'm certain that rebuilding SRPMS on architectures that were never considered in the original packages takes a great deal of skill, expense, time, and effort. You may have also noticed that CentOS placed 2nd out of all other distributions for Linux distribution of the year in Linux Journal Magazine. I also find it remarkable that CentOS provides rebuilds of RHEL 2.1, 3, and 4.
It is unfortunate that you are not satisfied by the tremendous efforts of the CentOS team.
I feel that the CentOS team does a remarkable job. I use CentOS on many servers and workstations to perform a wide variety of tasks. I have never once been disappointed.
Barry Brimer, RHCE