The last time I returned to ATL from CUN, there were huge lines at customs (ATL). I got in the shorter Global Entry line. By the time I got through the line, all the other lines were gone. Global Entry sucks compared to what it used to be.
YMMV, and it's all pretty random on any given day & airport.
My experience:
Nov 4
PHL -> DFW -> CUN -> ADO -> CZM
Left PHL at about 10AM
Customs in CUN -- there was a long backup in the lines for domestic (Mexican) entry, and the queue for US visitors was empty and closed. As I approached behind 2 other people, they opened the barricade, I walked up to the passport scanner, and then was almost immediately directed through immigration with no delay. Luggage took about 45 min. There was no delay for inspection (and no significant number of other passengers being inspected). The ADO bus was delayed by about 15 minutes due to an accident on the highway, so I missed the 7:30 ferry, got the 8PM, and was in CZM before 9.
Nov 14
Left CZM on the 10AM ferry -- rainy, choppy, but not terrible weather -- I purchased my ferry ticket the night before.
Heavy rain in PDC -- to the point where my luggage was soaked through during the few minutes of bag-drag, trying to stay under awnings.
I had pre-purchased the bus ticket in CZM (from the ADO storefront on Calle 2N between 5th & 10th) so I walked right past the line of people queued to buy tickets and boarded promptly. No significant bus delays despite heavy rain for the first 1/2 of the drive -- the 11:05 bus got to T3 by 12:20PM.
Outbound security took seconds. There was no check for the FMM QR code or Visitax.
Inbound Immigration in PHL was fast -- stand still for about 3 seconds for a photo then walk directly to baggage claim (thanks, Global Entry). No Customs paperwork or inspection. Home before 9PM.
Of course, this is only a single anectdode, not "data"...and it's a lot better than my last experience doing the same return route, where everything went fine until PHL, where they sent all luggage to a different carousel and didn't notify any passengers for about an hour (last international arrival of the evening).