Slow shifting is a bummer, and there are lots of reasons for it, but if you've eliminated derailleur problems, cassette issues, and general grime as the culprits, it might be time to replace your shift cables and housing with Shimano's Dura-Ace OT-SP41 Polymer-Coated Derailleur Cable. The housing is pre-lubed with silicone grease for smooth, stick-free shifting, and the polymer coating on the cable allows it run even more smoothly. Even the end caps are sealed, which prevents dirt and grime from sneaking in and gunking up the system.