Check out the [matg] tag (Game Globals), you can change crouching speed, sideways speed and etc.
The method above is for Halo PC. In the future to get the best answers you can, it would be prefable to specify which Halo version you are asking about.
do what he said but dont put it at 300 and a 1000 that would make you go through walls and die lol put normal speed 100 then crouching speed 50 or the other way around cause i ahve had my speed at 150 and i ran through walls and died