I think you have forgotten about Immigration.

the process is:

Deplane
Make way to Immigration
Wait on line (depends on how many planes have landed at once)
Clear Immigration
Go to baggage carousel
Wait for luggage
Go through Customs (showing your filled in form)
Check luggage for next flight
Go through security

If you bought this as one ticket and you don;t make it the airline is responsible to get you on the next available seat to your destination (no guarantee of when this is)

If separate tickets you ar eon your own and will have to buy a new one at the walk up rate.

Typically airlines want you to be checked in and at the gate 30 minutes before take-off - which means 1 hour after wheels down. I would think possible but by no means sure.

I am trying to remember where was our stop in the US on the way back from Costa Rica... maybe we were just lucky as I can't recall any long lines.

What I do remember is in the airport in Costa Rica they have a 2nd security check upon boarding. We went through the first security just like in the US, and I had an empty water bottle in the carry-on that I filled with water at the boarding gate.

They checked our carry-ons, and took that bottle away! I didn't mind explosives swab, and the wand, but I still want my beautiful bottle back

Thanks for the explanation on the customs/immigration process. I'm hoping we have enough time. It was all purchased as one to ticket so I'd think that United wouldn't give you a schedule if it didn't give you enough time to make your connection. Thanks for the Costa Rica info as well. I'll be sure not to have any water bottles

Yes, if you don't make your connection, United will put you on the next flight. Happens all the time, esspecially with tighter and tighter connections. And I am sure that there are many flights a day from Houston to LAX, so if you don't make your flight, you can get on the next one.

Carry-on luggage is helpful for tight connections. That way, you don't have to wait for luggage before you go through customs and then recheck it - saves time because as soon as you clear immigration you are on your way - the customs check is very quick. Also, you don't have to worry about ending up on a different plane than your luggage if you or the luggage don't make the connection.

If you do miss your connection, call the airline customer service number on your cell if there are a lot of people in line waiting to rebook. You can rebook your flight over the phone rather than waiting in a long line of people and then you just have to have a gate agent print your boarding pass.

Thanks. We are planning to only have carry-ons. I also thought that would help quite a bit. I just added United's number to my contacts just in case.