Does ECC SDRAM slow down a non ECC compliant system

hi. does anyone know if using ECC SDRAM in a non-ECC compliant system slows it down and by how much. i was surprised to find AMD says it does (at http://www.amd.com/support/faqs/athlonfaqs.html#7b ). i've used parity and non-parity ram in old non-parity macs for years and never had any problem or noticed any performance hit. the mac just ignored the parity bit. i thought it would be the same with ECC. it seems to me that since all the bits are transmitted on the same clock cycle, it shouldn't matter. apparently it does. anyone know? any response is appreciated. thanks. bye, roy