There is no doubt that reverser effectiveness IS higher at high speed, but it is difficult to find a thorough explanation in quickly-found documents. One relatively simple graph is in Figure 3 of
http://flightsafety.org/files/alar_bn8-4-braking.pdf. A simplistic answer may be that at higher forward speed, the relative [negative] velocity of the reversed airflow is also higher, imparting more [negative] force.