I just did JFK-LHR-FRA last Wednesday night / Thursday. Immigration into FRA around 11:30am was quick (helps that I was in the 2nd row of Club Europe on BA so
#3 off the plane, plus many passengers were EU citizens so went to a separate queue). You will need to go through immigration as a US citizen. There was no one else in line or in the baggage claim area that I could see, perhaps there were not many flights arriving into T2 at that time.
Once out of immigration and customs, I took the aforementioned shuttle bus. Not knowing there was a direct bus to the Fernbahnhof, I took the bus to T1. Up an escalator then a long walkway to the Fernbahnhof. Probably about 30mins all in from exiting the plane to arriving at the train station.