Reply back with complete model number.
Speed (mph) is a poor reference tool to use when troubleshooting. Simply because speed can vary to a great degree caused by numerous variables. Rpms is a more valued tool you'll want to focus on. 15mph does sound very low for a 200hp. However, speed is relative to the load the engine is pushing. For all I know your pontoon weighs 2 tons.
So, how many rpms are you seeing at WOT? I believe the spec rpm range for your engine at WOT is 4500-5500. I would guess from what little you've detailed that 15mph is not producing 5500. But I prefer not to guess.
90% of the time when an outboard is struggling it's related to the fuel system....whether its fuel, filters, hoses, primer bulb, carbs, etc. So I'd start troubleshooting there.
