With visibility set to 3 miles, you can't see most buildings in the bombsight of a B17 from 16,000 ft. With visibility set at the default (approx 1 mile) invisible ack guns will nail you at the airfield towns from distances that seem to be closer than 1 mile. Bomb strikes are invisible at all altitudes I've tried (10K to 16K)with vis set at three miles.
in B17 @ 10K, 4x AA, AF off, detail sliders 50%, Visibility 3.0 miles, any view, Frame rate = 85 (monitor refresh rate). Near the ground can breifly go as low as 30 fps depending on view.