Its simple really - if they are on the same cable they will both work at the slowest interface speed, so for example if your disk is ATA133 and the CD is ATA100, they both will run at ATA100, but this should be fine.
i would like to know where did this information come from.
by the way, the fastest optical drive i've seen until now was ata33, even modern dvd burners
See below... My experience comes from where I used to work for a computer manufacturer at around 1995 to 1996, I suppose things have changed a little....
Mixed device speeds
It is a common misconception that, if two devices of different speed capabilities are on the same cable, both will necessarily transfer data at the speed of the slower device. This is true only with very old chipsets or add-in adapters. All modern ATA interfaces (since, at least, the late Pentium III and AMD K7 era) support independent timing, which allows each device on the cable to transfer data at its own best speed.
However, due to the omission of both overlapped and queued feature sets from most real-world parallel ATA products, the preceding paragraph must be clarified. It applies to the data transfer phase, but this is usually the shortest part of a complete read or write operation. Devices do differ markedly in the total time required to perform an I/O (irrespective of the burst data transfer rate), and since only one device on a cable can have an operation in progress at one time, they do affect each others' performance.
For example, consider an optical device such as a DVD-ROM, and a hard drive on the same parallel ATA cable. With average seek and rotation speeds for such devices, a read operation to the DVD-ROM will take an average of around 100 milliseconds, while a typical fast parallel ATA hard drive can complete a read or write in less than 10 milliseconds.
This means that the hard drive, if unencumbered, could perform more than 100 operations per second (and far more than that if only short head movements are involved). But since the devices are on the same cable, once a "read" command is given to the DVD-ROM, the hard drive will be inaccessible (and idle) for as long as it takes the DVD-ROM to complete its read—seek time included. Frequent accesses to the DVD-ROM will therefore vastly reduce the maximum throughput available from the hard drive. If the DVD-ROM is kept busy with average-duration requests, and if the host operating system driver sends commands to the two drives in a strict "round robin" fashion, then the hard drive will be limited to about 10 operations per second while the DVD-ROM is in use... even though the burst data transfers to and from the hard drive still happen at the hard drive's usual speed.
The impact of this on a system's performance depends on 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.
On the other hand, it is worth emphasizing that this effect only occurs if the slow drive is actually being accessed. If it is idle, its mere presence on the cable will not affect the performance of any other device on the same cable (provided, of course, that the host adapter supports independent timing).