Chris, can you explain a bit further?
"Fan reversers do not deflect the air very much forward - much more sideways. This means they are reasonably effective at high speed, because the speed of the aircraft vectors the air forward. But they are less and less effective as you slow down."
Let's take a simplified case of the engine with the thrust reverser deployed producing a constant thrust during a portion of aircraft's deceleration (pilot sets and maintains appropriate N1). Perhaps not 100% true, but for aircraft speeds involved during landing seems ball park.
Assume the thrust angle of the exhaust is 60 degrees off of the direction of travel. Then surely the deceleration force is cos(60) * thrust, or .5 * thrust, regardless of the aircraft's forward speed. No? Is my example unrealistic for the aircraft deceleration once reverse thrust is established?