My bike has internally routed cable and the shifting seems OK, especially after I installed new shifter cables.

My guess is that the design of the bottom bracket shift cable guide is key to shifting performance. The one on my bike has large and gentle bends for the cable compared to some others which I have seen in these forums. But the guide has to be removed to thread either shift cable through the guide, and both cables have to be removed to remove the guide, so the design essentially forces me to replace both cables at the same time.