I read this in the manual from Plextor PX-712a, but don’t say why!!!
I don’t know EXACTLY why, but I would imagine it has something to do with transfer rates. The further the drive is away from the IDE Channel (not physically, but in terms of Master Vs. Slave), I would imagine the rates slow down. Also, if you can leave it as the sole drive on a channel, then it doesn’t have to share bandwidth to the mobo with any other drive (which is also in the manual, but unexplained).
Two drivers use one IDE (especially at SLAVE position)will reduce performance of CD/DVDROM or harddisk (especially of CD & DVD recorders)