First, those are Ian's tests, not mine (I lifted them from his review at http://www.cdrlabs.com/reviews/index.ph ... erformance
) I can't claim credit for the nice charts.
The test shown in my earlier post was for CD-RW media, which is generally harder for drives to read at speed than pressed and CD-R media. With pressed and CD-R media the Memorex/LTR-52246S will go to its maximum 52x speed.
I suspect that your results were caused by the notoriously poor quality of Memorex CD-RW discs. Here are some error tests of the Memorex CD-RW sample disc (10x in my case) that came with my Memorex 48MAXX drive (the following two charts are mine):
You can see how bad these discs are. Mine quit working altogether (can't be read/written/erased at all) after something like four erasure cycles. Your disc is probably starting to go bad as well, and when a disc goes bad the drive has to slow down to try to read the difficult sectors.
You can see a similar effect in Ian's test when he tested some 99-minute discs which, because they are out of the official CD specifications, tend to be more difficult to read as well:
You can see that at the end of the disc, where the higher speeds and greater wobble and vibration make any disc harder to read, let alone a 99-minute disc, the drive had to slow down to compensate.