I just flew SFO-FRA (UA900) and FRA-CDG yesterday (Wednesday). We landed in Hall B, although the plane landed in a remote airstrip and we had to take a bus to Hall B. The FRA>CDG flew out of Hall A. So first hope that your plane isn't landing with the bus, which of course you can't control, but if it happens, don't get too shocked (it's happened to me the last couple years I've flown on UA900, but I only fly it once a year). It really really sucks though even if not in a rush, especially if FRA is the final destination. More wasted time It offsets that the SFO>FRA flight tends to arrive "earlier" than expected.

We had 2.5 hours so weren't worried; we went first to LH SEN to get BPs for our CDG flight (as we were given 3 window seats in SFO and wanted 3 seats together), and after we received the BPs we were warned that A was still a 20-minute walk. Sure enough, it took some time to get from B to A. And there is still another security check to enter A, although you keep your shoes on and they don't care about liquids.

You need to know your exact gate number because within both A and B, there are different sections/pathways depending on the gate number.

It isn't pretty, but if you miss your flight I'm sure they'll put you on the next one. FRA is a pretty big airport which means a lot of foot traffic of people in front of you if you are in a rush.

But like Mahasamatman said, you would have customs in CDG... and that was a quick breeze. Immigration at FRA can be a slow line if you are a non-EU citizen and there are other non-EU citizens in line ahead of you... because we stopped at LH SEN lounge to get our BPs, there was nobody at all in front of us both for immigration and security.

But we went to the LH SEN lounge closer to our actual departure gate, and we left about 5 minutes too soon, and thus waited at the departure gate longer than needed. Gate lice also with LH PAX - gate lice everywhere