The Carbolite frames feature a steeper seattube angle and slacker headtube angle than the generation of bikes that came before, and I feel that the changes were for the better, especially as the chainstays also were shortened.

As kunsonoke mentioned, try moving the shift lever by slight increments, in both directions, with the bike stationary. The return action should be almost mirror-image of the take-up movement, much less and you are losing much shifting precision to hysteresis losses in the cabling (the combined/multiplicative effects of cable friction and elasticity).
Check the rear-most section of cable housing by tugging on the exposed wire along the downtube, again the return action should be without apparent friction. Lube the wire where it passes under the frame's bottom bracket if any friction is evident, as this is a common place for friction issues to appear.

The rear derailer looks like a very good-shifting unit, and rear derailers are very often blamed for shifting problems originating elsewhere. Most derailers do fine without lubrication other than what's needed to keep the pullies turning freely (another good thing to check).

I firstly suspect that the freewheel and chain are very much sub-optimal for best shifting performance. Modern chain and vintage Uniglide freewheel would be my first recommendation toward tactile and most-responsive friction shifting on any bike.