There is no real connection between the rated and/or burned speed of the media and the reading/ripping speed. On 16X drive, I burn at 4X, I can read up to the 16X. Also, don't forget that officially movies are limited up to ~10X speed.
Try using Nero CD-DVD Speed test program, and test few DVDs, try both data and video.
The max speed of 16X is only at the edge of the disc, assuming the disc is clean and without problems, otherwise the drive slows down to try and read the errors.
Pressed disc may run faster than burned, single layer run faster than dual layer, clean run faster than dirty/scratched.