Rider Fatigue and Bicycle Design
A reasonable assumption would be that a heavier and slower bicycle would be more fatiguing than a lighter, faster one, but for me that is not always the case. It seems to have more to do with how the bike feels on the road. When a bike does not do a good job dampening road shock, I begin to feel exhausted very easily. I also seem to be sensitive to a bicycle frame's tubing, because some bikes just feel more effortful to propel forward than others, despite similar geometry, size and fit. Oddly, positioning does not seem to have as much to do with it for me as these other aspects: Some bikes I can ride for a long time in an upright position and some bikes I can ride for a long time in an aggressive drop-bar position, whereas on other bikes these very same postures begin to feel exhausting sooner.
What has been your experience with fatigue on different bicycles? Have you noticed any patterns or connections? I suspect that there is no one formula to this. No doubt it is a complex interaction between a number of factors, including individual anatomy.