This is a weird problem to solve, because there could be several factors playing into the problem.
Weight alone shouln't affect top speed very much, but cross-sectional area of your body (which we can imagine goes up with weight) does. Are your tires at 20 psi or higher? Is your chain in decent condition and well lubricated?
And, is there any possibility that the previous owner changed the gearing from 15/45 to something lower?