A speed setting system that generates a speed control command for controlling the rotational speed of a stall-controlled wind turbine of a wind-powered machine. The speed setting system generates the speed command as a function of performance variation of the wind turbine due to environmental and/or other factors. The speed setting system utilizes a performance-compensation term that is slowly adjusted to compensate for relatively long-term performance variation. In one example, the performance-compensation term is adjusted only when the current power output of the wind turbine is at least 80% of the rated output power to ensure the wind speed is sufficiently high.