That’s not diminishing returns in terms of time and speed, which is CanadaPlus’ point. 100km/h faster is 100km/h faster, not 100% each time. It would be diminishing returns if doubling the speed each time didn’t halve the travel time, but “diminishing input = diminishing output”, or 100% -> 50% -> 25%, etc, is not diminishing returns, that’s linear.

An actual example of diminishing returns would be the cost to speed ratio, where doubling the budget each time will not result in a doubled speed.