Coin Magnet Performance Issue Mobile

FindGameObjectsWithTag each Update isn't a good idea, for sure. You could try OverlapSphere to find the objects inside the range, and keep the coins in some special layer (layer 8, for instance) to restrict the search (player script):

 var magnetRange: float = 5;
 var minDistance: float = 0.5;
 
 function Update(){
   var coins: Collider[];
   var here = transform.position;
   // find colliders inside range and in layer 8:
   coins = Physics.OverlapSphere(here, magnetRange, 1<<8);
   for (var coin: Collider in coins){
     var tfCoin = coin.transform;
     var dist = Vector3.Distance(tfCoin.position, here);
     if (dist < minDistance){ // closer than minDistance: grab the coin
       Destroy(tfCoin.gameObject);
       // add the points to your score
     }
     else 
     if (dist <= magnetRange){ // inside range: attract it
       var vel = magnetRange / dist; // velocity inversely proportional to distance
       // move coin to the player
       tfCoin.position = Vector3.MoveTowards(tfCoin.position, here, vel * Time.deltaTime);
     }
   }
 }

Only the coins should be kept in layer 8 - other objects in this layer would be attracted too.
NOTE: Since OverlapSphere only verifies the collider bounds, some of them may actually be a little out if range - that's why the distance is verified inside the loop.

Here it goes:

 //This is the script on player - the coin is only collected with an actual trigger collision.
 //And I'm more used to C#, if that is not a problem.
             
             int coinLayer = LayerMask.NameToLayer("coin");
             Vector3 here = transform.position;
             Collider [] tempCoins = Physics.OverlapSphere(here, magnetDistance, 1<<coinLayer);
             
             foreach (Collider obj in tempCoins)
             {
                 Vector3 coinPos = obj.transform.position;
                 float dist = Vector3.Distance(here, coinPos);
                 if (dist < magnetDistance)
                 {
                     obj.GetComponent<MoedaController>().getHookedByMagnet();
                     obj.transform.position = Vector3.MoveTowards(coinPos, here, Time.deltaTime * magnetForce * (magnetDistance / dist));
                 }
             }

//An this is the whole CoinController class

 using UnityEngine;
 using System.Collections;
 
 public class MoedaController : MonoBehaviour {
     
     public int coinValue=1;
     public float spinSpeed = 245;
     public float movementSpeed = 15;
     public float minDistance = 0.01f;
     public GameObject moedaEspecial;
     
     private Vector3 targetPos;
     private bool reachedPosition = false;
     private Quaternion myRotation;
      
     // Use this for initialization
     void Start () {

             //this is for a crazy random rotation behavior the coin has.
         transform.rotation = Random.rotation;//Quaternion.Euler(0, tempYRot, 0);
         myRotation = Random.rotation;
     
     }
     
     void Update () {
         
         //Rotates the coin
         transform.Rotate(myRotation.x * spinSpeed * Time.deltaTime,myRotation.y * spinSpeed * Time.deltaTime,myRotation.z * spinSpeed * Time.deltaTime, Space.Self);
                     
         //Initial movement of the coin to a random position nearby its spawn point
         if (!reachedPosition)
         {
             //move the object
             transform.position = Vector3.Lerp(transform.position, targetPos, movementSpeed * Time.deltaTime);
             
             if (Vector3.Distance(transform.position, targetPos) <= minDistance)
             {
                 reachedPosition = true;
             }
         }
     
     }

     //The object that handles all coin instantiation sends trough this method the range of the random position generation and another int that represents a small chance of this coin being a special coin worth 10x the original value.
     public void setUp(int [] _temp)
     {
         targetPos = Random.insideUnitSphere * _temp[1] + transform.position;
         
         if (Random.Range(0,100) < _temp[0]*3)
         {
             coinValue *= 10;
             transform.GetChild(0).renderer.enabled = false;
             GameObject go = (GameObject) Instantiate (moedaEspecial, transform.position, transform.rotation);
             go.transform.parent = transform;
         }
     }

     //This is maybe the source of the problem. The player calls this method for all coins within magnet range every frame. Without it the coins attracted by the magnet would have two movements at once and often get suck at a point in space.
     public void getHookedByMagnet()
     {
         reachedPosition = true;
     }
 }

And as far as I know somethings about programming, when you change something in one place, you can bet all your money that it will create other problems, I'm foreseeing one right now. The player is able to collect one "super-magnet" powerup that increases its magnet range and power for a time frame. When the code of the coin movement by the magnet get changed to the coin script, what will the best way to collect this data (stored on player?). Can't be GetComponent, as it would make this effort to improve performance useless.

Thank you very much for your help in this.

 public void magneticCoins()
 {
     float magnetForce = 1;
     float magnetDistance = 2;
     int coinLayer = LayerMask.NameToLayer("coin");
     Vector3 here = transform.position;
     Collider [] tempCoins = Physics.OverlapSphere (here, magnetDistance, 1 << coinLayer);
 
     foreach (Collider obj in tempCoins) 
     {

     Vector3 coinPos = obj.transform.position;
     float dist = Vector3.Distance (here, coinPos);
     if (dist < magnetDistance) 
     {
     obj.GetComponent<MoedaController>().getHookedByMagnet();
     obj.transform.position = Vector3.MoveTowards (coinPos, here, Time.deltaTime * magnetForce * (magnetDistance / dist));
     }

     }
 }

tha's it ?