I went through FRA twice in the past three weeks on different trans-continental flights, once on a weekday morning and once on a weekend afternoon. Both times the airport felt crowded but it took me no more than 20-30 min to go from landing gate (once a busing gate) through immigration to the departure gate ( A <-> Z). it is a confusing airport to navigate but you should be fine if you just follow the signs. As others have pointed out, TXL is a breeze, especially if you qualify for fast track.