Kalk is mildly abrasive. kalk sediment also turns into a paste. Depending on the orientation of the pump, IE the input/output section is pointing any way other then down, you may have sediment in the impeller cavity. That would make the impeller turn more slowly.
I have had maxi impellers separate over time. I am saying this from memory but looking at the impellor/magnet assembly hold it with the impeller at the top. there is a cap on the bottom side of the magnet. That cap has a tube that goes up through the magnet and is attached to the impeller. That cap is what has always failed in my maxi's. It simply separates at the cap side which causes the magnet to free wheel rather then drive the impeller.
Hope this helps.
You can get replacement impellers at premiumaquatics dot com.
Adam