"since you will be outside of security once you go through immigration."
I'm not absolutely sure on this because I have always terminated in Frankfurt when coming there from the US, but I think, as long as you don't change concourses (gate areas), that you can stay inside security. Every time I've flown into the A concourse in Frankfurt, immigration has been directly under the gate, and I think that after clearing immigration, I could have returned to the secure area of the concourse. Of course, that was on Lufthansa, within the same concourse. If you change concourses or terminals, all bets are off.
For passengers on flights that are leaving soon (about 30 min), FRA has created security and immigration fast lanes. There will be a board at the lane indicating the flights for which passenger can use the lane.