Lots of pilots and airlines often discuss this. It is a very interesting question.
The thrustlever idle-to-reverse time delay is most effectively used to manually deploy speedbrakes which also will serve as a back up for auto-speedbrake failures,
Possible malfunctions with auto-speedbrake or reveres have happened so many times that it is even factored in during certification. Keep in mind that during JAR or FAR certification the reveres are not used to determine stopping distance, they just use thrust levers idle and speedbrakes manually deployed.
Late speedbrakes will affect the stopping distance. Next time you are in the sim, try to slam the forward thrustlevers to idle and note how there is a slight delay before you can move the reverse thrustlevers. This will dealy auto-speedbrakes. The most time efficient technique is actually: (1) Thrust lever idle, (2) manual speedbrakes, (3) reveres.