“As yet unmeasured, but believed to be faster than light owing to its ability to move so quickly out of lights way.”
This is brought up in Terry Pratchett’s The Science of Discworld, when the faculty of the UU is standing in the university squash court wondering about the possibility of their new thaumatic engine (the Discworld version of a nuclear reactor, which they are using to heat the building) exploding, and the lack of events afterward i.e. TEOTWAWKI.
The speed of dark is the fastest thing in the universe. The idea is that since the speed of light is 186,000 miles per second, it must be terribly slow, since wherever light goes the dark is sitting there waiting for it.
There is still discussion over whether or not it exceeds the speed of lint.