My guess is also sand. This tends to get into the rotating part of the impeller, and can damage the magnetic surface, and prevent it from turning efficiently.
Also, check that the intake tube isn't split or broken anywhere. If it is, the filter may be working harder than it should have to to suck water up.
Lastly, make sure the sponges are thoroughly cleaned. If the water is not cascading easily out of the filter, the impeller may be working harder than it needs to.
is a blessing, not a disease.