Mercury 25 hp 4 stroke question

I just bought a 2005 tracker 165 with a mercury 4 stroke 25 hp garage keept, really taken care of. First run, put 9 gallons of 93 octane in it. Started great, fan smooth in neutral, with full range of rpm. Full smooth rev to 6500 7000 rpms. Once in forward gear ran smooth but only would reach 4000 at 8.53 mph. Three guys all weighing around 200 each plus gear. It seemed really slow to me, was expecting around 15 to 20 mph considering the weight. Still can't figure out why only 4000 rpms while in gear but full range when in neutral. Any thoughts, thanks

?---In neutral the motor does not have to do any work.----It would rev up like that on one cylinder.

I have a 16ft Lund SSV which is a lighter boat than yours by about half. I have a Mercury 25 EFI 2006. With 3 people in that boat the motor barely gets it on a plane. Probably around 15mph. By myself with the boat it can do about 20mph. I am a fishermen so honestly I really never go over 5 mph. But I am under the impression there is nothing wrong with your motor sir. I'm not an expert though. Could be low gas flow from the tank which could be as simple as opening a vent or it could be your not getting spark on on all cylinders. Is it surging or sputtering?

Cheers