[IainIreland]

"The score is a rescaled version of inverse time" is the key here.

If you run all the tests in half the time, your Speedometer score will double. If your score improves by 1%, it implies that you are 1% faster on the subtests.

(There are probably some subtleties here because we're using the geometric mean to avoid putting too much weight on any individual subtest, but the rough intuition should still hold.)