Heathpack 

So he says.

Definitely the higher cadence at the same power is more aerobically taxing for me. The trick to my high cadence in my TT last week was that my power was pretty low in the first half of my TT. I can do these higher cadences at lower power without much problem, but if I need to crank it up a notch, it feels way easier for me to do that at a cadence that's 10 rpm lower.

Since high cadence pedaling taxes me differently than does lower cadence pedaling, one strategy I revert to in a TT is to alternate between the two. I may not be able to sustain the high cadence stuff aerobically but I feel much more lactate in my legs with the low cadence stuff. I have a vibe for how much long my legs can survive the low cadence pedaling. So I frequently pedal faster early in a TT and when I can't do that any more I drop the cadence and crank out some power. I will pay the price for doing that but hopefully not until I've crossed the finish line. A 20K TT is short enough that I can get away with lower cadence and higher power.