Will not go fast as in engine won't spin up RPM or the engine spins fast but you almost aren't moving?
2 very different things...
Engine turning high rpm and "going nowhere fast" (not moving a lot of water):
Actually the prop can be slipping long before 2000 rpm . Once the rubber insert starts slipping a little at high load, it can progress to slipping all the time and eventually the prop just falls off. It doesn't take long from just some high speed slip to slipping any time the engine is in gar in the water.
I've seen a couple where you could have the outboard in gear and easily hand turn the prop without the engine turning.
Engine failing to get up rpm can be a LOT of things.
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Quick basic check for bent shaft. Put it in neutral and spin the prop. Watch the center of the shaft to see if it looks like its wobbling. No easily visible wobble and its PROBABLY not bent.
To be sure you have to pull the prop and use a dial indicator to check the shaft "runout"
If its bent... you won't have long before the bearings go out and you hear bad grindy noises.
