I think that even not taking things out of the bag has become less of an advantage for pre check. I flew through PHL & SAN a few months ago and didn't need to take liquids out of my bags at either airport. My DIL flies numerous times a year and hasn't had to do that in ages. And even having to take her laptop out, she still often gets through faster than her husband who has pre-check. I know a lot depends on the specific airport and the particular agents working at the time as well. And also how prepared the people ahead of you in line are.
It depends on the machines used and right now the machines that allow for nothing to be taken out of the bags were pulled back from more widespread usage because they significantly slowed down screening times so they are working on improving them. In Atlanta where they were using it it was causing multiple bags, including my husband, have to be rescanned with the agents having no idea why and saying it's not the bags themselves. The goal has been though for the tech to be there where people don't have to take much if anything out in normal lines as well as pre-check lines.
Like I said in my comment it isn't always about being the fastest. That is aside from the cases where there's such severe back ups some of which have been heavily publicized where pre-check was deemed a virtual necessity, think ones where the security lines stretched outside of the building. Flying out of JFK though not lines out the building pre-check has always been more advantageous for us there. Vegas is also another airport where pre-check has speedier on average for us.
Quicker at my home airport? No, not since they moved to a single terminal in 2023 with a single security checkpoint. However, when we had 2 terminals (used to be 3 but one stopped being operational years ago) the regular security was almost always 5-10mins at most with pre-check quicker than that. But that's because in the old terminals security was broken up covering multiple gates so it helped overall. Now they just have a lot of lanes available open at one checkpoint so it caused the normal lines to go down even further but creep up a bit on the pre-check side compared to before since it was previously spread out more. That said it's now takes an
eternity to get to the gates especially SWA at my home airport whereas you used to step from the curb to the security to gate.
My husband flies all the time though primarily to west part of the U.S. (Vegas, Phoenix, Denver, multiple places in CA like San Jose, Sacramento, Oakland, etc) because of his job and when he's with coworkers that don't have pre-check he's almost always done quicker and has an easier time through security.
Security procedures have always varied by airport with that being the intent. One thing to note is that outside of the light jacket part where I've had to remove them sometimes because everyone had to in pre-check (my home airport did that in September for example but didn't the any other time) you know that having pre-check means you don't have to do xyz, whereas without it it's more a toss up between what tech is being used, what the agent feels like that day, what the airport's procedures at the moment are, etc so it's more that you aren't just playing "what's it going to be like today" and more consistent throughout.
Like I said as things evolve pre-check will need to evolve with it. Each person gets it for different reasons but it's not
just for time savings, certain programs like touchless ID is only available to pre-check at the moment.
A notation for your son if he is seeing it often enough that pre-check isn't getting him faster and that's why he's using it there's really no reason why he is choosing to continue to go through the pre-check lane. It's not a requirement to go through there even though you have it. My husband had it before I did (because his work paid for it via global entry even though he himself doesn't really go international for it) and when I traveled with him he just went through the normal check out lanes with me until I got it via Global Entry. So if I were your son just go through the regular line in the future when he's with his wife.