Sorry in advance for the length of this post...
Most of the backups I could see in January, after parks went FP+ only (AK and MK saw last week), were more of the cast members explaining things to people, or some operational choices that slow things down.
As those photos show it gets really backed up at some attractions but I don't think it's only the 4 or 5 seconds to scan the band or ticket... Here's 2 examples.
1) CM explanations / guest education:
Having a MB or ticket alone does not get you in there, you have to book a FP+. I saw this play out numerous times on my trip right in front of me. Usually the family would move over to the stand-by line, and they understood quickly, but other times some wanted a detailed explanation or English is not their first language so CM spends a minute or two in that case explaining about that you had to book FP+. I saw this happen to day guests with plastic tickets and to resort guests with MB. After all this time the resort guests do not all get the whole MB and FP+ thing, so I have to forgive the day guests showing up and seeing FP+ for the first time! In a year I'd be more annoyed, but it's been only a WEEK at the MK.
2) CM making a choice to scan ALL the bands or tickets fora group:
Some FP return points, the CM would say "only one of you has to scan your band in the party" to move things along faster, saw that at BTMR more than once. Then other attractions or other times, CM would make every single person in the party scan their band or ticket. Saw that at Test Track multiple times (still had paper FP when I was there though).
RE #2 -- Either the training is uneven, or
CMs have realized some people are already "milking the system" (spreading their FP+ across multiple attractions for different people in the party). Just think with 3 people in your party, if you only had to scan one band or ticket at each FP entry, you could TRIPLE your FP+, in theory. So I suspect the CMs are (perhaps wisely) checking ALL of the people in a party SOME of the time, even when there's a backup. It's unfortunate but...
Checking only one ticket or band on a busy day, people are going to abuse the heck out of that loophole if it became a predictable norm that CMs always only check one ticket/band for a group. So I think the CMs have to intermittently make everyone scan, sort of how parking attendants check your ID vs. your AP about 10 to 20% of the time going thru the parking plazas.
They won't stop everyone but it will deter you if you got checked say one out of five times. And this is where in time, you will see guests have a fake "meltdown" and act outraged they are getting the blue light, and yelling "How dare you check all of us you only checked one of us last time?!" plus some more hystrionics, etc.
I don't like 3 FP or 1 park limit, and most others don't either -- so people are going to try all kinds of tricks to get around these 1 park and 3 FP limits. There are some other creative ways too, people getting around the FP limits it's going to be worse than the whole GAC card abuse thing I fear, if the limits don't ease up soon that is.
HINT: Those CMs with the iPads at the kiosks (or at service centers like Sid Cahuenga's) can do all sorts of things to bend and expand the limits on that system, and do "magical" things we cannot do with the kiosk or the MM+ app! Those limits we can see are only suggestions to a CM in a really good mood.