Strange crash, please help

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityStandardAssets.Characters.FirstPerson;
 
 public class AdditionalControls : MonoBehaviour {
 //    public GameObject target;
     public static bool sneak;
     public static float runBoost = 0;
     public static float baseRunBoost = 0;
     public RigidbodyFirstPersonController fpsScript;
 
     void Start() {
     
         runBoost = 0;
         baseRunBoost = 0;
     
     }
 
 
     void Update () {
 
         fpsScript.movementSettings.RunMultiplier = 1.5f + runBoost + baseRunBoost;
 
         if (Input.GetKey (KeyCode.LeftControl)) {
             sneak = true;
         } else {
             sneak = false;
         }
         if (sneak == true) {
             transform.localScale = new Vector3 (1, 0.1f, 1);
             fpsScript.movementSettings.ForwardSpeed = 2;
             fpsScript.movementSettings.JumpForce = 0;
             fpsScript.movementSettings.RunMultiplier = 0;
         } else if (sneak == false) {
             transform.localScale = new Vector3 (1, 1, 1);
             fpsScript.movementSettings.JumpForce = 40;
             fpsScript.movementSettings.ForwardSpeed = 7;
             if (fpsScript.movementSettings.Running == true) {
                 while (fpsScript.movementSettings.RunMultiplier < 2.5f) {            
                     runBoost += Time.deltaTime * 0.1f;        
                 }        
             }
         }
     }
 }

This is a script i have made for my character in my game. I want that when the player starts sprinting, it will take some time until the character reaches max speed. Now however as soon as i sprint, unity freezes and i have to "force quit" it. Help appreciated.