You may be correct. If the CD has a short duration the "end" speed will be smaller because the audio track starts from the inner edge and the maximum speed is reached on the outer edge. For this comparison it might be good to test a CD that has a duration of approximately 60 minutes. In any case, since the duration info should be included, the results will probably be comparable with other CDs that have approximately the same duration. The idea is to roughly see if a drive is fast or slow, not to produce scientifically valid results.
Otherwise my experience is different from yours. When I have ripped factory pressed flawless CDs the speeds have been quite consistent (assuming the durations don't differ much). Reread attempts may slow down the process so it is important that the log file doesn't show any rereads.
In addition, the CD should be a genuine Red Book CD without any copy protection system that may affect the speed.
Since each tester (I hope there will be more than one
) is testing a familiar drive or drives it should be easy to report also other related things, like any previously detected or newly found inconsistency in the ripping results. (However, please don't report bugs in this thread. Test only drives that work normally.)