EditorWindow to slow

Hello and excuse my bad english. I'm making an editor with EditorWindow. My target is to create a map (scene) like RPG Maker. They take a color (that's a 3D prefabe ... a cube) and draw it from top to bottom in the editor (or like paint). The editor makes this picture to a map.

The problem is currently the EditorWindow update too slow. When I draw a line, it takes a few seconds for it to appear.

Is it possible that EditorWindow is updated more often? Or does jamand have another solution. My code:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEditor;
 
 public class WorldEditor : EditorWindow
 {
     const int maxSize = 30;
 
     Field[][] world = new Field[maxSize][];
     List<Field> drawList = new List<Field>();
     GameObject ground;
     GUIStyle style;
 
     class Field
     {
         public int x;
         public int y;
         public int height;
         GameObject ground;
         GameObject environment;
         GameObject learnSystem;
 
         public Field(int x, int y)
         {
             this.x = x;
             this.y = y;
         }
 
         public void Draw(GUIStyle style)
         {
             if(ground != null)
             {
                 Rect rect = new Rect(x, y, 20, 20);        
                 GUI.Button(rect, "", style);
             }
         }
 
         public void SetGround(GameObject ground)
         {
             this.ground = ground;          
         }
     }
 
    
 
     private void OnEnable()
     {   
         for(int x = 0; x < maxSize; x++)
         {
             world[x] = new Field[maxSize];
             for (int y = 0; y < maxSize; y++)
             {
                 world[x][y] = new Field(x*20,y*20);
             }
         }
       
         ground = EditorGUIUtility.Load("Assets/Resources/Prefabs/World/Ground Grass Long.prefab") as GameObject;
 
         style = new GUIStyle();
         style.normal.background = EditorGUIUtility.Load("Assets/Resources/Editor/DialogSystem.png") as Texture2D;
     }
 
     [MenuItem("Window/World Editor")]
     public static void OpenWindow()
     {
         WorldEditor window = GetWindow<WorldEditor>("World Editor");
         window.Show();
     }
 
     private void OnGUI()
     {
         DrawBG(20, 0.2f, Color.gray);
         DrawBG(100, 1.5f, Color.gray);
 
         DrawField();
         Mouse();
     }
 
     void DrawField()
     {
         /*
         for (int x = 0; x < maxSize; x++)
         {          
             for (int y = 0; y < maxSize; y++)
             {
                 world[x][y].Draw();
                
             }
         }
         */
 
         for (int i = 0; i < drawList.Count; i++)
         {         
             drawList[i].Draw( style);
         }
     }
 
     void Mouse()
     {
         Event e = Event.current;
 
         if (e.button == 0)
         {
             if (e.type == EventType.MouseDrag)
             {
                 world[(int)e.mousePosition.x / 20][(int)e.mousePosition.y / 20].SetGround(ground);
 
                 if (!drawList.Contains(world[(int)e.mousePosition.x / 20][(int)e.mousePosition.y / 20]))
                     drawList.Add(world[(int)e.mousePosition.x / 20][(int)e.mousePosition.y / 20]);
 
               
             }
         }
     }
 
     private void DrawBG(float space, float transparence, Color color)
     {
        
         int currentWindowSizeWidth = (int)position.width / (int)space;
         int currentWindowSizeHeight = (int)position.height / (int)space;
 
         Handles.BeginGUI();
 
         
         Handles.color = new Color(color.r, color.g, color.b, transparence);
 
         //Height
         for (int y = 0; y < currentWindowSizeHeight + 1; y++)
         {
             Handles.DrawLine(new Vector3(0, space * y, 0), new Vector3(position.width, space * y, 0));
         }
 
         //Width
         for (int x = 0; x < currentWindowSizeWidth + 1; x++)
         {
             Handles.DrawLine(new Vector3(space * x, 0, 0), new Vector3(space * x, position.height, 0));
         }
 
         Handles.EndGUI();
 
     }
 }