I have the same boat and engine. Most, maybe all of the suggestions above are valid.
I would not discount black smoke at higher rpms. The underwater exhaust tends to mask black smoke. Black smoke is an indication of overloading, or not enough air getting to the engine.
Low speed at a given rpm almost always means a fouled prop or bottom or maybe a sudden increase in boat weight or as a longer shot, a change in calibration of the tach or knot meter.
Remember props move boats and engines only turn the prop. So if the prop is being turned by the engine at 2,800 rpm, then there is nothing wrong with the engine, it is doing what it should be doing.
The low speed has something to do with the prop or boat that is causing the lower speed than at the same rpm as before.
David