If the engine is off (ignition on) and battery voltage is lowish, it will move slowly, this is normal. If engine is running and it is still slow, and you're getting 14v or so (alternator is still good) I would suspect the pump is getting weak or fluid inside is dirty. I don't think those pumps are servicable though :/ I have heard that the pump from a 2nd gen copen works (not sure if it's a drop in replacement or needs something extra), so that could be a source of parts if needed.
