More than 20 instances of a certain script cause fps to skyrocket, is there something I'm doing wrong here?

Hi, I'm currently working on the AI for an RTS game I'm working on. One of the AI's is set to find the nearest target, move to it, then destroy it, then repeat. If it gets shot in this cycle however, it finds the nearest shooter, kills it, then continues to the original cycle. The AI works as intended, but not to any considerable scale. More than 20 of these AIs spawned results in a severe fps drop (not a problem for any other scripts, and I need to be able to handle up to 200. Here is the AI code, I'm not sure if there is any one issue, or if the whole script needs to be rewritten.

public class activeSoldierAI : MonoBehaviour { GameObject target; Target targettype; bool allclear = true; int hp = 50; int lastsavedhp; bool isShooting; Rigidbody selfBody; // Use this for initialization void Start () { targettype = Target.Building; target = ClosestTarget ("Building"); lastsavedhp = hp; StartCoroutine (shoot()); selfBody = GetComponent ();

     }
     
     // Update is called once per frame
     void Update () {
         Debug.Log (hp);
         Debug.Log (allclear);
         //Debug.Log (targettype);
         Vector3 forward = transform.TransformDirection(Vector3.forward) * 10;
         Debug.DrawRay(new Vector3(transform.position.x,transform.position.y + 1.5f,transform.position.z), transform.forward, Color.green, 15000);
         if(targettype == Target.Building){
             if(!Physics.Raycast(new Vector3(transform.position.x,transform.position.y + 1.5f,transform.position.z), transform.forward, 25) && target != null){
                 transform.LookAt (new Vector3(target.transform.position.x, transform.position.y, target.transform.position.z));
                 rigidbody.AddForce (transform.forward *  7 * 60 * Time.deltaTime);
                 isShooting = false;
                 selfBody.drag = 0;
             }else if(target != null){
                 isShooting = true;
                 selfBody.drag = 6;
             }else{
                 target = ClosestTarget("Building");
                 //Debug.Log("HOwde doo");
             }                      
         }
         if (hp < lastsavedhp && allclear == true) {
             targettype = Target.Soldier;
             target = ClosestTarget ("reserveSoldier");
             allclear = false;
         }
         if(targettype == Target.Soldier){
             if(target != null){
                 Debug.Log("YUR FUCKED MATE");
                 isShooting = true;
                 transform.LookAt (new Vector3(target.transform.position.x, transform.position.y, target.transform.position.z));
                 selfBody.drag = 6;
             }else{
                 targettype = Target.Building;
                 target = ClosestTarget("Building");
                 isShooting = false;
                 allclear = true;
                 lastsavedhp = hp;
                 Debug.Log("HOwde doo");
             }                      
         }
         if (hp <= 0) {
             Destroy(gameObject);
         }
     }
 
     IEnumerator shoot() {
         while (true) {
             if(isShooting == true){
                 if(targettype == Target.Building){
                     target.GetComponent<buildinghealth>().TakeDamage();
                     Debug.Log("Blammo!");
                 }else{
                     target.GetComponent<reserveSoldierAI>().TakeDamage();
                     Debug.Log("Blammo!");
                 }
                 //Debug.Log("Blammo!");
             }
             yield return new WaitForSeconds(1f);
         }
     }
     public void TakeDamage(){
         hp -= 10;
     }
 
     GameObject ClosestTarget(string tag){
         GameObject[] objectsWithTag = GameObject.FindGameObjectsWithTag (tag);
         GameObject closestObject = null;
         foreach (GameObject obj in objectsWithTag) {
             if (!closestObject) {
                 closestObject = obj;
             }
             if (Vector3.Distance (transform.position, obj.transform.position) <= Vector3.Distance (transform.position, closestObject.transform.position)) {
                 closestObject = obj;
             }
         }
         return closestObject;
     }
 }