Why does my rigidbody.AddTorque gets "limited" ?

I'm working with a RigidyBody attached to a barrel with a sphere collider, and using my movement script which is like this:

 using UnityEngine;
 using System.Collections;
 
 public class Movement : MonoBehaviour {
     private Rigidbody rb;
     public float movSpeed, rotSpeed, throttle, turn;
 
     // Use this for initialization
     void Start () {
         rb = GetComponent<Rigidbody>();
         movSpeed = 25.0f;
         rotSpeed = 15.0f;
     }
     
     // Update is called once per frame
     void Update () {
         if (Input.GetKeyUp(KeyCode.UpArrow) || Input.GetKeyUp(KeyCode.DownArrow)) {
             //Slower rotation when idle
             rotSpeed = 12.0f;
         }
         //In air shall it slow down rotation speed ???
 
         //Lockrotation
         rb.rotation = Quaternion.Euler(0.0f, rb.rotation.eulerAngles.y, rb.rotation.eulerAngles.z);
     }
 
     void FixedUpdate()
     {
         //Moving with keys
         throttle = Input.GetAxis("Vertical");
         rb.AddTorque(transform.forward * movSpeed * throttle, ForceMode.Acceleration);
         //steering
         turn = Input.GetAxis("Horizontal") * 1.2f;
         rb.AddTorque(Vector3.up * rotSpeed * 1.6f * turn, ForceMode.Force);
     }
 }
 

My problem is that, the speed gets limited to a certain amount: 2.5f What is limiting the speed of my transform ?

Thanks in advance !