1.8 m/s difference for spear cav is just about as big as the speed difference between spearmen and skirmishers. It doesn't make for much of an apparent difference because cav are generally on another planet in terms of mobility.
in another rts game, speed is very carefully balanced, with small differences in movement speed making substantial differences in perceived mobility:
I'm not saying we should copy this, its for juxtaposition against our situation. If spearmen and champcav in 0ad had the same disparity as pikemen and knights above, champcav would only go 12.8 m/s. I just don't think its good for the only counter to champcav to be 0.5 to 0.4 times as fast, but absolutely massacre them if they end up fighting.
We can experiment in the near term with palisades, walls, and buildings soon (as well as building walls over forests).