Hulk is fueled by nuclear power(forces of atomic destruction).
And nuclear power is this.....
Rupturing atoms at their core = gamma rays= Light-speed or faster than light-speed travel = The theory of relativity( E= mc²)
Hulk hits with the force of a nuclear arsenal. His muscle contractions move at the speed of light or greater to do that.
Because of this, the writers don't have to directly state Hulk's speed when he's really cutting loose with his striking/thrusting. .
The nature of gamma rays speaks for itself. Constantly stating Hulk's speed would just be redundant.
Here's of some scans of what i'm saying....
Hulk's power easily dwarfs any nuclear assault that earth can muster. Here he shown ripping through a bunker designed to withstand the explosion of many thousands of megatons in magnitude. "Against the force of the gamma-energized Hulk.... it fares not well at all in incredible Hulk #177:
And once again, The power of gamma rays on full display here as the Hulk's "gamma-spawned might" can also give light to an "eons-dark cosmos" in Incredible Hulk #126: