That sounds reasonable. I think that the third thing on my list is the culprit.
What I mean by that one is this: in order for the computer to rip the music quickly, the cd must spin around quickly in the drive, correct? I.E. when the computer is ripping at 7x, the disc must spin at least seven times as quickly on average as it does in a music cd player.
Compact discs are not perfectly flat -- they have some slight amount of warping. This warping has a significant impact on the drive's ability to spin the disc very quickly -- the more warped a disc is, the harder it gets to spin it at a high speed. It must work the drive motor harder, and there is more of a chance that the warpage will cause the bit on the disc surface to be mis-positioned so that the laser will not read it. When this happens, the drive detects it and slows down automatically.
For this reason, discs that are flatter will rip more quickly and ones that are more warped will rip more slowly....