I am still attempting to install an IDE hard drive as the slave to an IDE DVD drive.
I think I understand the jumper configuration required for the IDE hard drive as slave. That is all that is required except for some changes in BIOS.
However, I have been told this arrangement will slow down my system considerably, that it would be better to mount this hard drive in an external drive enclosure and use a USB connection. I have always thought that internal drives were much faster than external USB drives.
Placing a HDD in an external enclosure will definitely slow down its use, UNLESS you have a USB3 or eSATA connection system to the external. Either of those interfaces can move data faster than the HDD itself can work. But on a USB2 interface, you'll definitely see it slower than if it were internal.
It used to be that, when you put both a HDD and an optical drive on one IDE port, you were best advised to make the HDD the Master device, and the optical unit the Slave. This was because a few older optical units were poor at doing all the Master device functions. For today's equipment, that is almost never a problem, so I would not worry about it. What you propose should work.
I do wonder, though, why you resist changing the optical unit to the Slave role, and making the HDD the Master device. I involves changing the jumpers on both devices, AND using the correct connectors of the IDE ribbon cable. The END connector (Black) should go the the Master device, and the MIDDLE (Grey) connector to the Slave. You should realize that this does NOT limit your use of either device in any way. For example, you may have the optical unit set (in BIOS Setup) as the very first device to try to boot from, followed by a HDD unit. You can still set this when the optical device is a Slave. BUT you may have to re-set it that way, since the BIOS tends to remember which device is which by the port it is connected to.
The advice you have received suggests that it is concerned with data transfer speeds, and not whether the arrangement will work at all. It implies that all optical drives are slower than any HDD, so that putting them together on one IDE port will cause both of them to operate at the slower speed. That certainly can be true in many cases. However, look closely at the specs of each unit - particularly the ATA number. Usually this is something like ATA66, ATA100, or ATA133. These are data transfer (max) rates for the device. Check also the spec for the IDE port in your mobo. If the specs all match, you should not experience any speed degradation on the HDD. If the port and HDD specs are fast but the optical unit is not, then you could see a reduced performance. BUT the alternative of putting your HDD into an external case connected by USB2 would be comparable to about an ATA66 performance on the internal IDE port.