I meant to add later that there's such a variance between burners that I honestly don't know which one to believe anymore. When a 1640-burned disc is scanned (with PXScan/Plextools) in my Plextor PX712a, the results are quite different than the scan above. Good, but not as good as above. Yet, when I burn this disc in my Plex the 1640 shows horrible scans in CDSpeed while the Plex shows it to be a pretty good burn.
I'm not sure if I trust the 1640/CDSpeed combo with scanning CDs. I have a CDR with known "slow" sectors; every app (including file copying in Win Explorer) slows right down when reading the files in the affected area. Yet the scan on the 1640 looks perfect, and there's not even any dips in the Read Transfer test. The Plex, OTOH, correctly shows C2 spikes in that area and a slight dip in the read transfer graph.
I'm guessing that the difference is that the CDSpeed/1640 "samples" the disc whereas the Plextor reads everything on it. That would certainly explain how & why the C1 scan takes a significantly shorter time than CDSpeed's "scandisc" function (which, BTW, shows yellow for the aforementioned "slow" areas on that disc). It would also explain what crossg has noticed in DVD scanning: take the disc out, put it back in, re-scan. and you get better (or worse) QS scores.