It is interesting to note that all drives are not created equal. Many times a person may have gone shopping and wondered why a branded drive can cost $200 while a no-brand drive can cost $100 with the same speed rating. Since the market is so competitive, why would anyone want to sell the more expensive drive? Surely there are reasons. The person may think that the brand name may be what causes the price to sky-rocket. But if that were true, why does the company, which creates the more expensive drive not go under? Surely they must have something more in their drive that their competitor's. Probably the real reason would be a longer-lasting characteristic or a greater quality drive that the branded company sells.
There are many factors that play a part in determining performance levels and requirements as well. Caching is one; the use of a good-sized cache can "cover up" the poorer performance of a low-speed CD-ROM drive. Another is the fact that in many cases, data from the CD-ROM can be copied from the CD to the hard disc and used there. In this situation, the importance of the performance of the CD-ROM drive is greatly diminished.
For example, many games have many and/or large data files that are required while playing the game. However, most of these have multiple installation options, some of which will copy a significant amount of data to the hard disc. Since the hard disc is much faster than any CD-ROM, a PC with a fast hard disc and a slow CD-ROM drive can outperform one with a middling hard disc and a quick CD-ROM, in many situations. (Of course the drawback to doing a lot of big installations on games is that you lose a lot of disc space this way.)
Finally, remember that even though 24X CD-ROM drives are available cheaply today, there are millions and millions of older, slower CD-ROM drives already out in the market. For this reason, most manufacturers of software will ensure that the performance requirements for their titles are rather conservative. It is possible that a software company will put out a new package that requires a 10X or better CD-ROM drive, but if they do this they will give up a large segment of their potential market. Most software only requires about a 4X player or better, and in fact most software will work with 2X or better devices, albeit slowly.