The difference in air drag is the most significant factor. Fact, not opinion. Regular duty balls have less felt than XD, hence less air drag (friction). Note that a 120 mph serve will typically slow down to 90 mph just prior its the bounce -- this is a significant reduction in speed to to air drag. I'm certain that a noticeable/significant difference would be seen in the pre-bounce speed of a RD ball compared to an XD ball.
The bounce itself will also reduce the speed of the ball. However, I would not expect a RD ball to be significantly different from an XD ball in this respect -- unless the internal ball pressure and/or the rubber parameters of the 2 types of balls are different. After the ball bounce, there would be some further slowing of the ball due to air drag.
Some have speculated that and RD ball might be lighter than an XD ball. There may be a very slight difference, if any. According to the rules of tennis, the rebound height, diameter and the weight of an ITF/USTA-approved tournament ball must fall within a fairly narrow range. The allowed weight range for an approved ball is 1.975-2.095 ounces.
While the weight of a RD ball might be (slightly) lighter than an XD ball, it is the difference in air drag that is the most significant factor in the speed of the ball.
Last edited by SystemicAnomaly; 11-27-2012 at 04:25 PM.