[andrewla]

> speed basically borrows energy from time

This is close to the main intuitions of special relativity as a geometric theory. I would phrase is more as "speed borrows space from time". In general relativity (and special relativity if you look at it geometrically) every reference frame moves at 1 second / second in their own coordinates, but an observer in a different frame will see you move in their coordinates -- the 4-vector of your time / position keeps the same magnitude, so since you are moving faster in space, your time is moving slower.