All data being transfered from IDE device to IDE device has to go back to motherboard and out again regardless of how you have the devices configured. You had the right idea, but wrong reason.
Even the right idea isn't 100% right. Putting the source and destination drives on the same IDE channel won't necessarily slow down your burn for an on-the-fly copy. However, it isn't recommended if you intend to do a lot of on-the-fly copying because depending on your device and controller speeds it may do so.
There is no one "correct" configuration for everyone. The general rule of thumb for best performance is to have the source and destination drives (whatever they may be, CD-RW, DVD-ROM or hard drive) on separate IDE channels. Thus, if you won't be doing on-the-fly copying but will be doing a lot of copying from the CD-RW to hard drive and from the DVD-ROM to hard drive, then you ought to put the two optical drives on the same IDE channel, separate from the hard drive's channel.
While it was once true that putting a slow IDE device on a channel would force the channel to operate at that slow device's rate, it is no longer true with modern controllers and devices that support independent device timing.
All of these issues are addressed in this thread and many more:
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