

It is not linear but some sort of hyberpolic function as the OP is describing: double the speed and you halve the travel time, you move closer to zero travel time but never reach it. With a linear relation you would reach zero travel time at a specific speed point.
And that makes the function like ’y = 1/x’ which is not a linear function. No dark magic, just math. The gist is the proportial part, not just inversely.