Dimplex heater fan seems to turn very slow - Dimplex Heating & Cooling

In-heater fans are usually driven by open-framed synchronous motors that gather dust and their sintered bearings dry out and because the motor isn't powerful the increased friction causes them to run slow.
The fan blades also tend to gather a layer of dust that isn't helpful in maintaining performance.

Performance is usually restored after removing the motor and removing accumulated dust, cleaning and washing the grime from the bearings with solvent and applying machine oil to the felt surrounding the bearings and a drop or two into the bearings.