This item is from the Yet Another Enhanced IDE/Fast-ATA/ATA-2 FAQ, by John Wehman and Peter den Haan with numerous contributions by others. (v1.92).
This is not necessarily the case. Still, it is generally preferable to connect older drives and CD-ROMs to the secondary channel.
If this is not feasible, or if you're wondering if you should upgrade, a few points.
o The speed loss usually referred to is in the interface timing, i.e. the speed at which the devices communicate with the computer. This does not necessarily translate into a real world performance penalty.
o This is mostly an issue with older ATA-2 (EIDE) interfaces and some VL IDE ones. If you have an ordinary ISA IDE interface, it can't get any slower.
o All modern interfaces support distinct timing for master and slave. With these, the slow device does not directly affect the fast one.
o Many CD-ROMs support at least PIO mode 3. This is enough to operate most harddisks on the market today near their maximum speed.
You can use Coretest <ftp://ftp.rahul.net/pub/lps/ disk/core303.exe> to determine if and how performance is affected; see Q4.15 for a recipe.
OS/2 and Unix users have another reason to put slow ATA devices such as tapes and CD-ROMs on a channel of their own. As long as one unit on a given channel is executing a command, the other is inaccessible. A CD-ROM can easily occupy the channel for 300ms that way.