If you haven't experienced any heavy steering with your boat before, it is very likely the rudder doesn't actually go down fully, while you think it does. A couple of cm (c.a. an inch) make a huge difference as soon as your speed increases; but this is hard to see when the boat is not on the dry. Your rudder should go up and down freely. It could have been damaged or even slightly twisted or more commonly the hydraulic cylinder is out of order (see Hydroem). This could stem from a corrosion issue within the cylinder or more simply some air in the circuit ...
The white plastic bushings (rudder stock or shaft) may also need to be replaced when worn out, easy to check, except the third one under the hull when the boat is in the water. However i would first try to find out why the rudder doesnt go up freely and down fully.
Fair winds, Kev