I frequently transit FRA, and there's a few things going on. First, the T-2 D gates that BA typically uses only have two lanes with scanning equipment. So, instant bottleneck there. Contrast that with the T-2 gates that AA, DL, or EK use which have much larger capacity and several lanes. Second, the security staff are utterly incompetent and thoroughly indifferent to the issues inherent in the security setup and their actions which exacerbate the issues. Frankfurt security is altogether bad, however when each passenger requires five or six trays due to their ridiculous policies and the scanner operator is the typical minimally capable buffoon its slows the throughput way down. The conveyor belt post-scanner gets backed up because the aforementioned buffoon flags every other tray for additional screening and nothing advances. Also, calling the priority lane by that name at these BA gates is an utter falsehood. Its given no priority. (That said, if its any consolation the A, B, & Z Gold track security for *A at T-1 is also staffed by buffoons and is equally as bad at times.)

My suggestion is to allow plenty of time for T-2 regardless, as you cannot predict when the passport lanes will be backed up and likewise with security. I have seen the passport queue stretch all the way back to the escalators for the sky train. Definitely in the morning when the bank of US bound flights are queuing along with the morning EK flight to DXB. Afternoon and later in the evening sees the other two EK flights. Smack in the middle of all this you've got gobs of other non-Schengen long haul happening. If you can use the E-gates for EU passports or are one of the three countries that can use Ez-Pass at the E-gates you'll have much better luck. However you could then run into a jam at the gate area security. There is no perfect answer. Allow too much time and you sit at the gate area with nothing to do and little to no amenities. Not enough time and you're in a massive jam. Its a crapshoot.

The most recent news is that T-2 will be idled in 2026 for modernization as T-3 is fully brought online. Maybe the madness will eventually end.