I have seen a bag line every time I have gone for a few months. A few weeks ago, I started taking pictures of it every time I enter, since its presence has been continually questioned. I'm not saying it's always there... but it's always been there when I arrive, which is on varied days of the week at varied times of day. I went through Harbor security nine times in May.
I also usually "time" my security wait times by taking photos as I walk up to the line and then once I get through, giving me an accurate timestamp of my actual wait time (I do this for a lot of things). It does indeed average 15-20 minutes in the mornings. In May, "morning" arrivals of 1 hour + or - rope drop ranged from 7 minutes to 24 minutes. I have indeed waited 40 minutes for Harbor security (not in May, though), and I've also been fortunate on a few occasions to have been in a long line when another table opened up, circumventing a long wait for me (but as mentioned, making for a rather inequitable experience for others).
As soon as the metal detector crowd backs up to the tables, they wave people through without it, but as noted by PP, that's a bummer for the last group left waiting there!
Slight side note but relevant: I often find that perception of "time waited" is very different than reality. On the PixarFest media day, I was in the RSR FP return line next to one of the media guests (identified by her lanyard and bag of swag). The FP line was VERY backed up, practically to the scanners, and she was tut-tutting the whole time and taking notes on her phone. When we got up front, she said, "what, that was about a 25 minute wait?" It had been 13, which I knew because we'd scanned in at the first moment we could, which meant I knew the time precisely... but here's a person related to media in some way, who easily could have published that the RSR FP return took her 25 minutes... when it really took 13. So I'm very wary of people's comments about how long they wait for stuff.