Well, there may be a problem with the engine. We have a Tohatsu 6hp 4 stroke for our 9 ft AB RIB and I always thought it was slow.
A few weeks ago we were using it on long stretches on flat water, versus the usual short hops to the boat in the mooring field, we were going wide open then all of a sudden the RPMs increased and the boat took-off. It has not done that before and not done it since. Why would it do that? Is there an adjustment? Is something clogged?