CentOS 6 was not really intended for slower systems, none of the newer distros are. CentOS 6 kernel for example does not support 586 CPU's.
I'd like to know where you read that because I'm looking at putting CentOS 6 onto a couple of lower end boxes, specifically a P3-800(mobile) and an older VIA EPIA, which are fully supported in CentOS 5.