Changing Mesh Vertex Colors in editor

I am trying to build an editor tool which will change the vertex colors of a chosen Mesh asset according to my calculations

I have the following code:

 using UnityEditor;
 using UnityEngine;
 
 public class GenerateVertexColors : EditorWindow {
     private Mesh[] meshes = new Mesh[4];
 
     [MenuItem ("MGD SS App/GenerateVertexColors")]
     public static void DoGenerateVertexColors () {        
         GenerateVertexColors window = (GenerateVertexColors)EditorWindow.GetWindow (typeof (GenerateVertexColors));
     }
     
     void OnGUI () {
         for (int i = 0; i < meshes.Length; ++i) {
             meshes[i] = EditorGUILayout.ObjectField (meshes[i], typeof (Mesh), false) as Mesh;
         }
 
         if (GUILayout.Button ("Generate")) {
             //MeshFilter[] meshes = modelGameObject.GetComponentsInChildren<MeshFilter>();
             float min, max = 0;
             
             max = float.MinValue;
             min = float.MaxValue;
 
             for (int i = 0; i < meshes.Length; ++i) {
                 Mesh mesh = meshes[i];
                 if (mesh == null)
                     continue;
 
                 for (int j = 0 ; j < mesh.vertexCount ; ++j) {
                     if (mesh.vertices [j].y < min) {
                         min = mesh.vertices [j].y;
                     }
                     if (mesh.vertices [j].y > max) {
                         max = mesh.vertices [j].y;
                     }
                 }
             }
 
             for (int i = 0; i < meshes.Length; ++i) {
                 Mesh mesh = meshes[i];
                 if (mesh == null)
                     continue;
 
                 Color[] colors = new Color[mesh.vertexCount];
 
                 for (int j = 0 ; j < mesh.vertexCount; ++j) {
                     float value = (mesh.vertices[j].y - min) / (max - min) + 0.1f;
                     colors[j] = new Color (value, value, value, 1);
                 }
 
                 mesh.colors = colors;
             }
         }
     }
 }

It only works if I import the materials for the chosen .fbx and if I set them to a shader that uses vertex colors. I tried doing this in runtime, but I have meshes with 40K+ vertices, so it is kinda slow.

Please help.