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?
I am well under the rated load for the bike and think it should do more. Tires are set at the specified pressure for max load. Chain was neglected but had been lubricated and seems fine now. I doubt the gearing has been changed based on the sellers comments but anything is possible. We'll see what the compression test shows tonight. From my reading it should be somewhere near 170psi or so new right?