Good afternoon Geoff, I assume this relates to a cooker hood.
If the motor is sluggish this could be a number of reasons, the most common is insufficient air circulation.
Start by looking at the outlet, for hood connected to a carbon\re-circulation filter, the filter may need to be cleaned\replaced, or if using a external vent the vent may have become blocked.
Once the vent \ filter have been checked, the next step is to clean the grill which sits directly below the hood, a lot of grease can block up the pours and cause issues with air getting into the fan.
If these have both been checked, and there is no improvement the fault lies with the motor itself, and the only option is to replace it, as the motors are not serviceable\repairable components.
Please let me know if I can be of any further assistance.