Oraeon did a ton of research on this.
Check out these two threads:
http://www.operationsports.com/forum...explained.html
http://www.operationsports.com/forum...do_oraeon.html
The more you raise threshold, the less separation in speed you will see between players. When you have a really low threshold, your running game benefits - especially if you have a speed back who can turn the corner easily.
The lower you go with pass D reaction time, the FASTER they react to the ball. The higher you go, the SLOWER they react to the ball.
With speed threshold and the passing game, Oraeon believes the higher you raise the threshold, the easier you will find passing against speedy corners (because their speed advantage is lessened). But I feel you would also have an easier time passing to a RB/TE that's matched up against a LB.