Issues with movement in an RTS Game.

After two years of programming in Unity with a couple of friends, I had to move away from them. Becaus of this and the fact that I had to leave our project, I decided to start my own. I decided it would be an RTS because of my love for the genre. I have, however, run into an issue. When I tested the script below on one of my basic units it worked fine. However, my problem is the fact that when I spawn more units the Game's framerate suddenly drops. I don't even have to spawn many units for this to happen. Often five is the amount that is needed before the game becomes completely unplayable. I hope some of you can help me with this, and would appreciate it if you could. My friends and I also had the same issue back when I was coding with them.

Here's the script. I have tried deactivating the Navmesh.

using UnityEngine; using System.Collections;

public class villager_behaviour_1 : MonoBehaviour {

 //Villager_components
 private GameObject villager;
 //Villager_movement;
 [SerializeField]
 private bool selected;
 [SerializeField]
 private float speed_move;
 private float standard_speed;
 [SerializeField]
 private float speed_rotate;
 [SerializeField]
 private Vector3 newposition;
 private float villager_y;
 private NavMeshAgent agent;
 private bool leftclick_exit = true;

 void Start ()
 {
     //Villager_components
     villager = this.gameObject;
     //Villager_movement
     newposition = villager.transform.position;
     standard_speed = speed_move;
     agent = villager.GetComponent<NavMeshAgent>();
     villager_y = villager.transform.position.y;
 }

 void FixedUpdate ()
 {
     agent.speed = speed_move;
     agent.angularSpeed = speed_rotate;

     if ((Input.GetMouseButtonDown(0)))
         if (leftclick_exit == true)
             selected = false;

     RaycastHit hit_screen_position;
     Ray ray_screen_position = Camera.main.ScreenPointToRay(Input.mousePosition);

     if (Input.GetMouseButtonUp(1))
     {
         if (selected == true)
         {
             if (Physics.Raycast(ray_screen_position, out hit_screen_position, 500))
             {
                 newposition = hit_screen_position.point;
             }
         }
     }
     agent.SetDestination(newposition);

     newposition.y = villager_y;      
 }

 void OnMouseEnter ()
 {
     leftclick_exit = false;
 }

 void OnMouseDown ()
 {
     selected = !selected;
     
 }

 void OnMouseExit ()
 {
     leftclick_exit = true;
 }

}