Procedural mesh redrawing textures causes in game fps drop. Is there a way to optimise this?

I have a procedural mesh drawing a grid of floor textures on in a 3D environment. The player can change the tiles by drawing on them. I am experiencing a significant drop in fps just doing the updating of the new textures. I know enough to understand that this is creating a lot of work, but not enough to understand how to optimise this. I added 2 boolean's that just flag instead of running the update in one whole chunk and this has helped out with about a 10% reduction in the lag it's all causing. Any help would be greatly appreciated. I feel that there should be a more efficient way to run the UpdateMesh as it feels like it's doing more work than it would need to, just to update the new floor tiles.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class BuildGrid:MonoBehaviour {
 
     public int floor = 0;
 
     public bool buildPending = false;
     public bool updatePending = false;
 
     List<Vector3> newVertices = new List<Vector3>();
     List<int> newTriangles = new List<int>();
     List<Vector2> newUV = new List<Vector2>();
 
     List<Vector3> colVertices = new List<Vector3>();
     List<int> colTriangles = new List<int>();
     int colCount;
 
     Mesh mesh;
     MeshCollider col;
 
     float tUnit = 0.1f;
 
     int squareCount;
 
     void Start() {
         mesh = GetComponent<MeshFilter>().mesh;
         col = GetComponent<MeshCollider>();
         buildPending = true;
     }
 
     void FixedUpdate() {
         if(buildPending) {
             buildPending = false;
             BuildMesh();
         }
         if(updatePending) {
             updatePending = false;
             UpdateMesh();
         }
     }
 
     void BuildMesh() {
         for(int px = 0; px < FloorManager.Instance.width; px++) {
             for(int py = 0; py < FloorManager.Instance.height; py++) {
                 GenCollider(px,py);
                 GenSquare(px,py,TextureMap(StoreManager.Instance.GetStoreColor(FloorManager.Instance.floorLayout[floor,px,py])));
             }
         }
         updatePending = true;
     }
     void UpdateMesh() {
         mesh.Clear();
         mesh.vertices = newVertices.ToArray();
         mesh.triangles = newTriangles.ToArray();
         mesh.uv = newUV.ToArray();
         mesh.RecalculateNormals();
 
         newVertices.Clear();
         newTriangles.Clear();
         newUV.Clear();
         squareCount = 0;
 
         Mesh newMesh = new Mesh {
             vertices = colVertices.ToArray(),
             triangles = colTriangles.ToArray()
         };
         col.sharedMesh = newMesh;
 
         colVertices.Clear();
         colTriangles.Clear();
         colCount = 0;
     }
 
     Vector2 TextureMap(int _number) {
         return new Vector2((_number - (_number % 10)) / 10,_number % 10);
 
     }
 
     void GenCollider(int x,int y) {
         colVertices.Add(new Vector3(x,0,y - 1));
         colVertices.Add(new Vector3(x,0,y));
         colVertices.Add(new Vector3(x + 1,0,y));
         colVertices.Add(new Vector3(x + 1,0,y - 1));
 
         ColliderTriangles();
 
         colCount++;
     }
 
     void ColliderTriangles() {
         colTriangles.Add(colCount * 4);
         colTriangles.Add((colCount * 4) + 1);
         colTriangles.Add((colCount * 4) + 3);
         colTriangles.Add((colCount * 4) + 1);
         colTriangles.Add((colCount * 4) + 2);
         colTriangles.Add((colCount * 4) + 3);
     }
 
     void GenSquare(int x,int y,Vector2 texture) {
         newVertices.Add(new Vector3(x,0,y));
         newVertices.Add(new Vector3(x + 1.0f,0,y));
         newVertices.Add(new Vector3(x + 1.0f,0,y - 1.0f));
         newVertices.Add(new Vector3(x,0,y - 1.0f));
 
         newTriangles.Add(squareCount * 4);
         newTriangles.Add((squareCount * 4) + 1);
         newTriangles.Add((squareCount * 4) + 3);
         newTriangles.Add((squareCount * 4) + 1);
         newTriangles.Add((squareCount * 4) + 2);
         newTriangles.Add((squareCount * 4) + 3);
 
         newUV.Add(new Vector2(tUnit * texture.x + 0.01f,tUnit * texture.y + tUnit - 0.01f));
         newUV.Add(new Vector2(tUnit * texture.x + tUnit - 0.01f,tUnit * texture.y + tUnit - 0.01f));
         newUV.Add(new Vector2(tUnit * texture.x + tUnit - 0.01f,tUnit * texture.y + 0.01f));
         newUV.Add(new Vector2(tUnit * texture.x + 0.01f,tUnit * texture.y + 0.01f));
 
         squareCount++;
     }
 
 }