I suspect it is slow because of design AND being light both. Also, check your other bows and see if they are heavier than marked. As some above mentioned...it is common for some to mark their bows lighter than they really are, as doing so makes their bows look fast. I think that trend is starting to change since people are now more educated. Many of Dan Quillians bows were actually only marked in poundage and draw length wasn't mentioned. People often assumed that mark was at 28", and while that may have been the case at one time, I have seen many that were actually the marked weight at a 26" draw length.
Additionally, some bowyers use a measuring tape on a wall and then pull the bow down to the mark on the wall...THAT IS THE WRONG WAY to measure bow weight. The distance to the mark is taken up not only by the bow, but also by the MOVING hook on the scale. I am amazed by the number of bowyers that do not notice this.
Imagine you have two bows...one 40# and another 70#. Pull them both to a mark on the wall...and the 70#er has pulled the hook on the scale down further since it is heavier...and therefore the bow itself isn't drawn as far. For this reason, when checking the weight of a bow, put a "measuring" arrow on the string with inch marks on it. You very seldom see that done, but it is the only way I know of to get a TRULY ACCURATE draw length/bow weight.
The only other way I know to get an "semi-accurate" draw is to have a "holding block" on the grip and then pull string to a mark. To do that though, the scale can't be anchored (often done with a pully and rope). The problem with this method is the grip variations will have to be taken into consideration as the distance to the back of the shelf in front of the "grip" block may vary in some designs.