I take it this is a
HOB type model, in most cases when they slow down like that, you just need to buy some filter brushes and clean the intake tubes out real good, you would be surprise what clings to the walls of the tubes even after a good washing in the sink. you may wanna make sure the impeller and everything is clean also, next try replacing the bag, its a good rule of thumb to replace them about every 2 months for optimal filtration, hope this helps