Also from Chicago, flew this summer (May/June 2022) fr ORD to ATH, ATH back to ORD, connecting in Frankfurt both ways. Booked through UAL, but flights were operated by Lufthansa. FRA is a Lufthansa hub, we thought that meant things would go smoothly...
From ATH back to ORD (on our way home), our layover in FRA was 85 minutes, but United assured me when we ticketed that was a sufficient layover. Maybe preCovid? Maybe not a weekend? Maybe this agent had never actually been in FRA airport.? Short capture, LH was 3 hours 39 min late getting to FRA, we missed the Chicago flight and had to stay overnight in FRA.
But here is the point: it was not just our plane, there were hundreds (thousands?) of people deplaning in FRA who had missed connecting flights because of delays (it was a beautiful, sunny day, no weather issues). Remember, you land and then taxi, it takes time to get off the plane itself and then get through the crush in the terminal. If one plane is delayed, that gate is occupied when the next plane arrives so there is more delay. There were security and passport checks. (And our luggage was checked through to ORD already.) It eats up time even when you arrive on time. This is especially true because FRA is a giant hub, people are changing planes, not a lot of people leaving the terminal in FRA so it is crowded.
FRA is huge (an opinion from someone flying in and out of ORD all her life!) We studied the airport map in advance and it gates looked deceptively close. They are not. The bigger issue is the lack of staff due to (we were told) Covid.
Go to flight stats, look up your flight, scroll down to see the on time performance. Would highly (highly) recommend extending your layover in FRA, from now on we are planning on a three hour layover there, at least until the staffing shortage eases. You can see my post on our experience in FRA: