if the fan is direct,mean the thermostat switch has been bypassed or removed, then running slow is may be due to motor problem. If some electrician has played with ur wiring then also check any loose connection for motor. Some time due to improper joint in wires. motor can not get required current so runs at low speed.
If fan is automatic, then check thermostat switch contact resistance. Due to corrosion at contact, may be motor is not getting required current.
Finally, think to replace motor.