Editor Extension + Lots of GUI Buttons

I am developing an Editor Extension that utilizes a grid of buttons. This grid can be extremely large at times, like 50 x 1000 buttons. I have wrapped the grid into a scrolling container, so only a portion of the grid is shown at any given time, but I'm trying to figure out a way to prevent the unseen buttons from drawing entirely.

Here is a sample of what I'm trying to do:

Code (CSharp):

1. using UnityEngine;
2. using UnityEditor;
3. using System.Collections;
4.  
5. public class SampleSlowdown : EditorWindow
6. {
7.     private Vector2 _scrollPosition = Vector2.zero;
8.     private int _rowCt = 10;
9.     private int _newRowCt = 10;
10.     private int _colCt = 10;
11.     private int _newColCt = 10;
12.  
13.     // Add menu item named "GoogleFu" to the Window menu
14.     [MenuItem("Window/SampleSlowdown")]
15.     public static void ShowWindow()
16.     {
17.         //Show existing window instance. If one doesn't exist, make one.
18.         EditorWindow sampleWindow = GetWindow(typeof(SampleSlowdown));
19.         sampleWindow.title = "Sample Window";
20.     }
21.  
22.     void OnGUI ()
23.     {
24.         _scrollPosition = EditorGUILayout.BeginScrollView (_scrollPosition);
25.  
26.         for (int row = 0; row < _rowCt; ++row) {
27.  
28.             EditorGUILayout.BeginHorizontal();
29.             for(int col = 0; col < _colCt; ++col) {
30.                 GUILayout.Button( row + " - " + col, GUILayout.Width(80), GUILayout.Height(25));
31.             }
32.             EditorGUILayout.EndHorizontal();
33.         }
34.         EditorGUILayout.EndScrollView ();
35.  
36.         EditorGUILayout.BeginHorizontal ();
37.         EditorGUILayout.LabelField ("Rows: ");
38.         _newRowCt = EditorGUILayout.IntField (_newRowCt);
39.         EditorGUILayout.EndHorizontal ();
40.  
41.         EditorGUILayout.BeginHorizontal ();
42.         EditorGUILayout.LabelField ("Cols: ");
43.         _newColCt = EditorGUILayout.IntField (_newColCt);
44.         EditorGUILayout.EndHorizontal ();
45.  
46.         if (Event.current.type == EventType.Repaint) {
47.             _rowCt = _newRowCt;
48.             _colCt = _newColCt;
49.         }
50.     }
51. }

Simply copy this script into an Editor folder and run it by opening the Window->SampleWindow
You can see that small grids, like the default 10x10 works just fine.
If you change the rows to something like 1000, the editor window becomes very unresponsive.