Smoothly rotate object based on GetAxis

Hello. I was wondering how I could rotate an object (rigidbodied) smoothly based on joystick axis.

 using UnityEngine;
 using System.Collections;
 
 public class InAirTest : MonoBehaviour {
 
     //raycasting variables
     public bool offGround = false;
 
     //rotation variables
     public float speed = 100;
 
 
    // public float tiltAngle = 30f;
    // public float smooth = 100f;
   //  float tiltX = Input.GetAxis("Horizontal");
   //  float tiltY = Input.GetAxis("Vertical");
    // public GameObject bike;
 
     // Update is called once per frame
     void Update () {
         RaycastHit hit;
 
         Vector3 down = transform.TransformDirection(Vector3.down) * 2f;
         Debug.DrawRay(transform.position, down, Color.red);
 
         /* if (Physics.Raycast(transform.position,(down), out hit))
          {
              distance = hit.distance;
              Debug.Log("Distance: " + distance + hit.collider.gameObject.name);
          }*/
 
         //testing if off ground
         //Quaternion target = Quaternion.Euler(tiltX, 0, tiltY);
 
         if(Physics.Raycast(transform.position, down, out hit))
         {
             if(hit.collider.tag != "ground")
             {
                 offGround = true;
             }else if(hit.collider.tag == "ground")
             {
                 offGround = false;
                 Debug.Log("...on ground......");
             }
         }
 
         //tell console if off ground
         if(offGround == true)
         {
             if(Input.GetAxis("Horizontal") > 0)
             {
                 transform.Rotate(0.0f, Input.GetAxis("Horizontal") * speed, 0.0f);
             }
             if(Input.GetAxis("Horizontal") < 0)
             {
           
             }
             Debug.Log("in air :D");
         }
     }
 }
 

This is the code I have so far. It rotates the way I want it to but it is delayed by 1.5 seconds roughly because of the conditional statements I believe also it seems to jump to the next angle. I also dont like how it doesnt gradually progress in velocity of the turn. Any hekp would be appreciated. Thanks.