no power at 6000 rpm

So here is the issue. With 9 people, coolers and crap, I couldn’t get above 15 mph at 6300 RPM’s (2 times out on the lake) However with 7 people, coolers and crap, I can Get 28-35 (choppy water pending and also, only one time, first time we had it on the water with a load since we bought it)...