Speedometer measures web app responsiveness. Roughly, it simulates a series of user operations on web apps built with various frameworks (as well as vanilla JS), and measures the time it takes to complete them and paint the results to the screen.
The score is a rescaled version of inverse time - if it goes up, that implies the browser can handle more user operations per second, or alternately, it takes fewer milliseconds to complete a user operation in a complex web app.
We know that, but you haven't said anything specific about scores other than higher scores are faster, in an abstract sense, which has already been established.
"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.)
The score is a rescaled version of inverse time - if it goes up, that implies the browser can handle more user operations per second, or alternately, it takes fewer milliseconds to complete a user operation in a complex web app.