You don't want actual grease in that long a run. It acts too much like a viscous shock absorber and slows down the cable movement just as you've found. Oil is the way to go. And it doesn't take much of it either. A light coat of oil will provide a nice snappy feel compared to greasing the cable. Clean it out and try the oil. Or better yet spring for some new lined housing and a stainless smooth drawn cable.