I had the same problem. There was a lot of solidified grease around the shaft that the gun turns on.
Locate the motor/gearbox assembly beneath the playfield, that moves the gun. That entire assembly is held in by four screws. Remove those screws and pull that assembly away from the bottom of the playfield. You'll reveal the shaft of the gun which comes down through the playfield. There is a C clip holding it in place. Remove the C clip and you will be able to lift the gun out from the top side of the playfield. I had to turn and pull pretty hard to get it out of there because of all of the grime. Clean and lube that shaft (using steel wool, rubbing alcohol and white lithium grease) and the hole that it lives in. Once it's clean, the gun will slip back in very easily and you should notice a big difference in the speed. But I warn you, it's much harder to hit those targets when the gun moves like it should.