How to optimize my short layover in Frankfurt? (RIX -> FRA -> ORD) US citizen, United/Lufthansa

Hello!

In 3 weeks I will be returning to the USA from a work trip in Latvia, with a 1.5hr layover in Frankfurt. What can I do to make my layover in Frankfurt as quick as possible with respect to customs and passport control? Any programs that I can sign up for as a US citizen? My layover will be between 7-9am on a Saturday so I am assuming it will be somewhat busy. I will have checked luggage but I expect that will go directly to my destination.