How increase boat motor speed?

So i just bought my boat recently and I'm trying to make the boat accurate faster in the open water. There are two throttle control lever, one for the idle/rpm and one for the forward/reverse which is gas. The motor is a 65hp mercury 4 stroke outboard. So when the idle/rpm and forward/reverse lever is in middle the propeller idles and don't move. When i push the idle/rpm lever forward it increase rpm speed making the propeller spin faster while it in neutral and if i pull it back it slow the idle rpm speed making the propeller spin slower. But when i have the idle/rpm at full forward and my forward/reverse lever at full forward to. My boat doesn't seem to go fast for a 65hp. It feels like super speed walking on water. The motor stars up fine on the first try and so on. But the boat doesn't increase in power to go forward. What am i doing wrong here can anyone help me? I try playing around with both the throttle control lever but it still doesn't seem to go fast. Also the boat is a 15ft fiberglass.