Doing a mini MR next week to CUN from SFO (excellent I fares on AA) and I had a choice between a 2-hr and a 4-hr turnaround at CUN. Am I right that there's no airside transit and I'll have to go through Mexican immigration and customs, then re-enter the secure area?

I guessed the latter and planned on the 4-hr turn just in case things get bogged down (Monday noon arrival.) Good choice?

Depending on how many flights arrive at once, immigrations and customs can take some time. Re-entry through security is fast. What takes a long time is check-in with luggage, which won't apply to you.

Not sure about staying only airside, as I haven't tried it.

I think you could have done the 2-hour turnaround, assuming it's the same plane you came in on. On the 4-hour turn, you will have time to eat something or step outside.

I haven't tried staying airside either, but I would opt for the 4-hour turnaround as the customs wait time can vary. Generally, it's only about a 15minute wait. However, if you arrive just after other flights, the lines can be quite long (45min+).

Replying to my own thread - did it today.

I rebooked for a (supposed) 3 hr layover; however AA did a last-minute equipment change at MIA and we had to wait for a reserve captain certified in type to get to the airport, so we were 1h 10m late departing, making my 3-hr turnaround closer to 90 min.

There's no airside transit; had to go through passport control (10 min.) then join the long customs lines, around 35 min (a couple of Thomson heavies landed bang-bang) for customs, outside, up to the AA agent who had to reissue my BP (I got both at MIA earlier) then up and through screening (Euro style, shoes on, computers stowed) and got to the gate 10 min. before boarding.

Moral to the story, 2 hrs would be scary, 3 good, but have a backup plan.

Good to know, with recent health break outs, it may be quicker now?