I have a new video DL DVD which I ripped with Decrypter in 16.5 mintues on a NEC 3540. I ripped it in Full Disc mode with Shrink in 19.5 minutes. So far so good. I could not better those times with the Sony/LiteOn I returned to the store - aka "The Coaster Maker".
I wiped the disc on my t-shirt to remove a finger smudge on the very edge. My t-shirt is clean from this morning but it is the slightest bit damn compliments of living in a subtropical swamp.
Shrink demanded 1.5 HOURS to rip that DVD. I then wiped the disc with a clean piece of cotton cloth. Now Shrink was back to its usual 20 minutes estimate.
The moral of this story is that although you may think the disc is clean, but the surface could have contaminants which radically effect the NEC 3540's reading performance.