It moves fast enough that the temperature delta between first and last chip isn't significant. Not too fast, though, because if the pump is running full-tilt it becomes a major heat-source itself.

> but you want it to be significant, otherwise it's less efficient.

No? You want a large delta between heat-source and water, as well as water and whatever is on your cool side. Essentially just between hot and cold sides.

Delta between heat-sources is irrelevant unless you start cherry-picking some very odd combinations.