* Windmills are designed for optimal performance at different wind speeds: The blades of a windmill are designed to spin at different speeds depending on the wind's strength.

* The rotor speed is not a constant: Windmill blades don't spin at a steady rate like a car's wheels. The speed varies depending on the wind.

* "Speed" can refer to different things: Are you asking about the rotational speed of the blades (measured in RPM), the speed of the electricity generated, or something else?

To give you a better idea:

* Rotor Speed: The rotational speed of the blades can range from a few revolutions per minute (RPM) in light winds to tens of RPM in strong winds.

* Electricity Generation: The speed of electricity generation depends on the wind speed and the efficiency of the turbine. This can vary greatly between different windmills.

Instead of asking for an "average" speed, consider these questions:

* What is the typical operating wind speed range for a windmill?

* What is the maximum RPM of the blades for a specific windmill model?

* What is the average power output of a windmill in a certain location?

These questions will give you a more informative understanding of how windmills operate.