I had a similar problem with one of my old steel frame road bikes. It has a steel cable route guide for the shifting cable brazed onto the to top of the BB. It shifted cleanly, but just difficult. This was due to the friction of the bare cable against the cable guide. I figured that I needed to do something to eliminate the friction. There is plastic tubing you could buy, but I ended up using the tubing from the guts of and ink pen since I only needed like 3” of tubing. It was just the right diameter and length. I cleaned the tube out thoroughly, and used it over the cable to line the cable guide. Problem solved. — Dan