The value is "distance to the farthest client" which is one-way. That it is actually round trip time is taken into account internally. I didn't think that setting less than 2000 meters have any effect versus default. If the one-way distance is more than 2000 meters you should increase it from the default. For short links that decreases performance because the radio will needlessly wait longer for ACK packets.
Radio waves travel at the same speed regardless of the radio frequency. This setting does not depend on the frequency.