Movement speed of player is much slower in build than in the editor

So I'm making a small runner game where the player can only move up and down to avoid obstacles.

When in the editor within unity, the player moves at the correct variable of speed of 175; yet when select file>build&run, the player moves a lot slower (at about 100) while the rest of the level moves at the correct speed.

I have no idea what could be causing this. I've tried resetting the player to the prefab, i've tried rebuilding it, i've tried reloading unity and i've tried editing it in the inspector and none of it has worked.

It's like this on all computers I try it on

     public float movementSpeed = 175f;
     private Vector3 movement = Vector3.zero;
     CharacterController playerMovement;
 
 
     void Start ()
     {
         playerMovement = GetComponent<CharacterController>();
         movementSpeed = 175f;
     }
 
     void Update () 
     {
         movement = new Vector3 (0, Input.GetAxis ("Vertical"), 0);
         movement *= (movementSpeed) * Time.deltaTime;
         playerMovement.Move(movement * Time.deltaTime);
     }

an example of the code.

If you can help at all, I would be greatly appreciative. This has been driving me mad

(I'm not the greatest programmer ;_; )