I think speed is more so effected by the noise on the cable rather than the length. Typically, a traditional flat IDE cable performs better than the cheap round ones because all the individual cables are bunched tightly together without proper shielding. The better made ones like the Antec Cobra series don't seem to have this issue. In regards to the really long cables, I've never had a problem using them with HDDs, but some CDROMs/DVD-ROMs don't seem to like them.