Welcome to the UTV Board!
the top speed is supposed to right at 25mph.
First off, whenever i have this similar problem with any engine, i start by running some carb cleaner through it (pour proper amount or whole can in gas tank) i usually do half a can for 4 gallon tank. and if that don't work
i would check your valve clearances, check your manual for specs.
Someone more experienced than me should come along soon. and will probably have a better idea than me.