I am flying from USA to Frankfurt, have a 2 hour & 10 minute layover in the airport until my connecting flight to Porto, Portugal. Do I have to go through customs in Frankfurt (in which case is 2 1/2 hours too short?) or can I go direct to my gate and go through customs at the Porto airport?
You will go through customs in Porto.
(But immigration is done in Frankfurt.)
In Frankfurt head to your gate. On your way you pass through immigration to the Schengen area.
In Porto you retrieve your checked luggage (if any) and proceed through customs (for goods). Declare any goods (red channel) or do not declare (green channel) and walk through.
Remember -- customs and immigration or passport control are two different functions even when returning to the US.
Plenty of time if it is an actual connection (booked on one ticket).
Transferring at FRA
Arrival at Frankfurt Airport: For up-to-the-minute information on your
connecting flight, please consult one of the monitor screens located
around the airport. If no gate has been assigned yet, head for the
indicated pier. Then follow the blue signs to the gate while keeping
the walking times displayed in mind. Gray signs show the way to
passenger services.If you are in a hurry to catch a connecting flight at Frankfurt
Airport, you can take a shortcut to get to your gate faster. The fast
lanes at the security checkpoint may be used on a priority basis by
passengers whose flight is listed on the display.Security Check
In most cases, you must clear security checks on your way through the
airport. Please allow enough time for them. Also please observe the
restrictions on carrying liquids and gels into the aircraft cabin.Passport Checks
Whether or not you must clear a passport check depends on whether the
airport you have arrived from and your destination are within or
outside the Schengen Area.You are arriving from an airport in a non-Schengen country and are
traveling to a destination within the Schengen Area: You will probably
have to clear a border check.
This is a double-post. You posted the same question under "Transport". I answered there: https://community.ricksteves.com/travel-forum/transportation/customs-and-layover-in-frankfurt-airport-1b0608d6-e88a-48f1-927f-790edd1903f2
I travel through the Frankfurt airport a lot and you should have plenty of time to go through immigration & security.