It is known that Nero CD-DVD scan of 1620 sometimes experiences periodic spin down and spin up. Most people see it at 12x and 16x scan, but weirdly, at 8x scan only a fraction of people including me reported it.
Today, I found the speed drop disappeared (Compare scan 1 and scan 2 of the same disc, look at the green line). The first scan is done just after its burn, the second scan is done a few minutes ago. As I guessed, the scan is a little bit better without the speed drop; I believe the error is reported higher just after the speed down-up.
Unfortunately, I don't know what caused such a change. There have been a few events between this and my last scan with spin down-up. One week gap, a few burns, a few flashes, etc. I even flashed it with a disc in it. (Yeah.. people do dumb things.)