Thanks for the suggestion, but that's not it. In the screenshot there are 13 equally spaced CVs along a motion path, for an animation 100 frames long.
Anything attached to this path covers a distance across the first 10 CVs in the first 30 frames, and then slows right down and takes 70 frames to travel the last 3 points.
It's definitely some kind of bug related to being close to the poles.