Lag when updating large amounts of objects

Hello! Working on an atmospherics simulator for a tile-based game. If a tile has a "leak" (either missing a diagonal/adjacent wall or floor tile), it drains the "oxygen level" of that tile (a float). I'm updating these values every five seconds based on the average of the values of all adjacent tiles. Unfortunately, I'm getting a lag spike every time it updates these values. Currently, I'm only trying to update a 9x9 grid of tiles (81 total) with one "leak" at one corner. The goal I'm trying to achieve works beautifully, it's just lag that's the issue at this point.

Two-part question for you all..

1. Is there any general procedure/guidelines to follow when updating large amounts of objects.

2. Is there anything specifically you could recommend for this script that would alleviate the lag issues?

Thanks in advance!

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class AtmosPropagation : MonoBehaviour {
     Collider[] adjacentObjects;
     List<GameObject> adjacentTiles;
     List<GameObject> adjacentWalls;
     public float oxygen;
     float averageOxygen = 0f;
     bool leak = false;
     // Use this for initialization
     void Start () {
         adjacentTiles = new List<GameObject>();
         adjacentWalls = new List<GameObject>();
         adjacentObjects = Physics.OverlapSphere(transform.position, 1.5f);
         if(adjacentObjects.Length == 0)
             Debug.Log ("NO COLLISION!");
         foreach(Collider col in adjacentObjects){
             if(col.gameObject.tag == "Floor"){
                 adjacentTiles.Add(col.gameObject);
             } else if(col.gameObject.tag == "Wall"){
                 adjacentWalls.Add(col.gameObject);
             }
         }
         oxygen = 20f;
         
         // [START ATMOS SIMULATION]
         InvokeRepeating("PropagateAtmos", 5.0f, 5.0f);
     }
     
     // Update is called once per frame
     void Update () {
         
     }
     
     public void PropagateAtmos(){
         CheckForLeaks();
         foreach(GameObject go in adjacentTiles){
             averageOxygen += go.GetComponent<AtmosPropagation>().oxygen;
         }
         if(leak == false)
             oxygen = averageOxygen / adjacentTiles.Count;
         //yield return new WaitForSeconds(5f);
         Debug.Log("Oxygen Level of Tile @ " + transform.position.ToString() + " = " + oxygen);
         averageOxygen = 0f;
         
         if(oxygen >= 18f)
             renderer.material.color = Color.green;
         if(oxygen >= 15f && oxygen < 18f)
             renderer.material.color = Color.yellow;
         if(oxygen > 10f && oxygen < 15f)
             renderer.material.color = Color.red;
         if(oxygen <= 10f)
             renderer.material.color = Color.black;
     }
     
     public void CheckForLeaks(){
         if((adjacentTiles.Count + adjacentWalls.Count) < 10){
             leak = true;
             Debug.Log ("Walls: " + adjacentWalls.Count);
             if(oxygen < 0)
                 oxygen = 0;
             if(oxygen != 0)
                 oxygen -= (oxygen * .1f);
         }
     }
 }
 

Edit: To clarify, this script is on each of my "floor" tiles.