I'm with Nino on this one.  Stuff like silicon spray will be a quick fix but as dirt and crap sticks to it, it will slow down again and then become a pain to clean.  I have always been told that machines don't need any lubrication.  As Nino suggested, change coil sleeves and clean the whole assembly, I'd say there is a good chance the plunger will have crud on it which will reduce it's power.