60hp Mercury command thrust...low power during WOT

[Davidvc1964]

I have a 2017 Sun Tracker Bass Buggy that has a 60hp Mercury 4 stroke command thrust engine. Approximately 1 months ago while out boating the engine suddenly started to sputter and would barely idle. After trying to get back across the lake with the throttle wide open the boat sputtered it’s way across the lake. Eventually, the sputtering cleared up and the boat ran fine. However, i decided to bring it in for its regular service and told the mechanic of the sputtering issue I had recently had. They serviced the boat, changed the oil and the fuel filter and all was fine. When I got the boat in the water, I noticed that during WOT the boat wasn’t going as fast as it used to. I was only getting around 4800 rpm as opposed to the usual 5400. Although the motor sounds and runs good, it just doesn’t have the same power it used to. I don’t know if maybe they retarded the timing or what. They also DID NOT change the spark plugs. I am not at all familiar with boat engines, so I’m hoping someone can give me some advice.