Fun kag scripts and codes

You need to modify the RunnerMoveVars, the variables you want are probably walkSpeed, jumpMaxVel, jumpFactor, overallScale and stoppingForce but feel free to tinker with all of them. You'll need to include the right script, get() the structure out of the blob, modify the values and set it back into the blob - and you'll need to make sure that it's done on all clients and the server or you'll get desync, as the structure isn't automatically synchronised.

You can check Entities/Characters/MovementScripts/RunnerMovementInit.as for the vanilla values. If you wanted to modify the speed of all players on the server, this would be the place to do that.

Spoiler: Explanation of what you're doing wrong above

The velocity of the object is it's current, as-of-this-frame change in position. If the object is standing still on flat ground, the velocity should already be practically zero (close to it, I think in practice gravity will make it have a slightly positive y velocity which will be resolved by the collision routines). However, when running at full tilt (again, on flat ground, not jumping), it'll have a negative or positive x velocity. Setting it to zero will only stop it for that frame - it'll get back up to speed momentarily.

Perhaps more fundamental: you're also creating a totally new object and plugging the player into that, instead of modifying the old one. An object that has just been spawned already has a zero velocity.