When your friend said the boat felt heavy you think he meant a large force on the tiller?
Look up sail balance. What it means is the force on the front of the boat is much the same as on the back, so it doesn't keep trying to go off course. You can let the tiller go to see which is out of line. Properly balanced the boat will more or less steer itself. There are several ways of adjusting the balance which can vary in different conditions.
A lot of older boats have pretty basic rudders. Without knowing how yours is designed it is difficult to comment. Some trailer sailers had a hinged rudder and if this was not straight it would be very heavy on the helm.
It will perform better if it is aerodynamically shaped, by rounding off the front third and tapering the rest slightly without weakening it too much.
Forget fibreboard. Before you change anything try to see what other owners have done. It won't be the weight of the rudder that is a problem because it is supported.
In the meantime have a look at a basic sailing book, and have fun while you experiment with trim and sail balance.