Taken from the wiki.
Two devices on one cable — speed impact
There are many debates about how much a slow device can impact the performance of a faster device on the same cable. There is an effect, but the debate is confused by the blurring of two quite different causes, called here "Lowest speed" and "One operation at a time".
 "Lowest speed"
It is a common misconception that, if two devices of different speed capabilities are on the same cable, both devices' data transfers will be constrained to the speed of the slower device.
For all modern ATA host adapters this is not true, as modern ATA host adapters support independent device timing. This allows each device on the cable to transfer data at its own best speed. Even with older adapters without independent timing, this effect only applies to the data transfer phase of a read or write operation. This is usually the shortest part of a complete read or write operation.
 "One operation at a time"
This is caused by the omission of both overlapped and queued feature sets from most parallel ATA products. Only one device on a cable can perform a read or write operation at one time, therefore a fast device on the same cable as a slow device under heavy use will find it has to wait for the slow device to complete its task first.
However, most modern devices will report write operations as complete once the data is stored in its onboard cache memory, before the data is written to the (slow) magnetic storage. This allows commands to be sent to the other device on the cable, reducing the impact of the "one operation at a time" limit.
The impact of this on a system's performance depends on the application. For example, when copying data from an optical drive to a hard drive (such as during software installation), this effect probably doesn't matter: Such jobs are necessarily limited by the speed of the optical drive no matter where it is. But if the hard drive in question is also expected to provide good throughput for other tasks at the same time, it probably should not be on the same cable as the optical drive.
That being said, If it were me, I'd buy a new sata DVD, they can be had from Newegg for about $20 free shipping. Cable about $3 or by a retail kit that has the cable, OEM does not. At least until hdd prices come down. But unless you are burning a disc, the same time you are hitting the hdd hard, there will be negligible speed impact on a normal day to day use unless you are a gamer, but then why are you still using IDE for any devices. And if you are going to mix types on the same cable, never choose CS, always set the hdd as Master, the odd as slave.