We have had these ceiling fans for about 9 years. They worked great at first at very high speeds. But now even on high they are going as slow as on low speed. I have a wall switch for the fan beside the light switch. What could be causing this?
To me, the fact that all the fans (you did not state how many there are) are slow probably indicates something other than the fan motors. Check to see if they are overly warm or hot to the touch, which could indicate a voltage problem. Turn off the circuit breaker or fuse and check the switch that controls them. You could wire the hot lead and switched-hot to the fan together temporarily (bypass the switch) and see if anything changes. I have encountered fans with light dimmer switches being used to control the speed, which will damage the switch and fan motors. Look upstream.