On an airplane like this rudder is required when using ailerons. When you have a large amount of di/polyhedral yaw will cause a roll. When you apply aileron (especially if you have increased the throws to get a better roll) you will get adverse yaw which will induce a rolling force counter to the initiated roll, reducing the roll rate. If you counter the adverse yaw you will maintain the full roll rate, increase the yaw just a little more and you will see a small increase in rate. As stated earlier, excessive rudder will increase rate but make it more of a lazy snap roll. Practice coordinating your rudder and aileron and see if it helps.