Mouse sensitivity changes between editor and built exe

function Update ()
{
    var x_delta = speed * Input.GetAxis ("Mouse X") * Time.deltaTime;
    var z_delta = speed * Input.GetAxis ("Mouse Y") * Time.deltaTime;
    transform.Translate (Vector3 (x_delta, 0, z_delta));
    Screen.lockCursor = true;
}

Script is attached to a sphere which can be moved about using the mouse. While playing in the editor the sphere moves at a reasonable speed - but after building, the mouse sensitivity (relative to moving the sphere) becomes very low (sphere moves at about 1/5th speed). I have tried different resolution/quality settings but none seem to affect the mouse sensitivity.