First and foremost, have you had your motor scoped for the dry exhaust corrosion issue that many early 2000's 4 stroke Yamaha's are known to have? From the research that I've done, it seems like it's more oftentimes found in the F225's but there are many reports out there of owners having issues with F150's, F200's and even F250's. If it's feeling sluggish or struggling to make power or get up to WOT, then you might want to drop the lower unit, look up and see what it looks like inside the motor. How long have you owned the motor for? Did you buy it brand new and did you put all 500 of those hours on the motor? When you say that it feels slow, does that mean that it feels slow getting up onto a plane or that it won't top out at WOT or how exactly does it feel slow? Also, does it feel slow now compared to the last time that you had the boat out or has it felt like this for awhile? Have you changed the boat up at all since when it last felt "normal"? The last time it felt normal was the fuel tank low and then you filled it all the way up? The maximum amount of horsepower for your year and model of boat is 280hp, and given that your boat is a 1990 that's going to be 280hp with a 2 stroke motor, which will be lighter than a 4 stroke. Being 80hp shy of the max along with having a heavier 4 stroke motor as opposed to a 2 stroke is not helping you out any.
On top of that, the stock fuel tank for your boat is 93 gallons, of which a gallon of fuel weighs about 6lbs/gal, so it makes a huge difference in weight if the tank is full to the brim or running on fumes. If you have a full tank you'll have roughly 560lbs worth of fuel in the boat. Comparatively though, if you only had say 1/4 of a tank or less you'd be at roughly 140lbs or less, which is a difference of over 420lbs. A weight savings of 420lbs just in fuel is going to be quite noticeable, especially if you're underpowered to begin with, and on top of that, if you had any other people or other gear with you on board the last time as well. It unfortunately might be something to where you need to only run the boat at half a tank of fuel or very minimal weight in terms of fishing gear or the amount of people you have on board with you. Obviously don't skimp on safety gear like life jackets and so forth, but what is the boat mainly used for? I see fishing rods in the first picture, so I'm assuming you use it mostly for fishing? If you're looking to remedy the situation, I'd first look possibly into trim tabs if you don't already have them as they'll help you get up on a plane more easily. Then possibly look into getting a different sized/pitch prop if it's still feeling sluggish and/or you want a better hole shot. My thinking is you probably added a bunch of fuel from the last time you used it and added a considerable amount of weight to is since last time. But I could be wrong, although 500 hours isn't exactly very much for a 4 stroke motor, especially one that's now 20 years old?