Iterating LList in OnGUI(), slow?

I am trying to come up with a solution for accessing and iterating through a linked list inside of the OnGUI loop. The performance takes a massive hit which is nearly impossible to use. I would prefer to keep my inventory as is without too much redesign but I imagine that working through the list each time the OnGUI calls is the problem.

I have tried a interval (every half second) and only creating the inventory once with a bool. Both of which have severe problems..

The below code is inside OnGUI()

                         //inv count number
                             int inv_count = inventory_script.count;
                             
                             // start at head
                             inv_item current_item_to_display = inventory_script.head;
                             
                             for (int c = 1; c <= inv_count; c++) {
                                 //Debug.Log ("for loop, adding item");
 
 
                                 // gui label for each inv item
                             GUI.Label (new Rect (inv_x, inv_y + (c * inv_h), inv_w, inv_h), "x" + current_item_to_display.quantity + " "  + current_item_to_display.id_name + " $: " + current_item_to_display.value);
                                 
                                 if(GUI.Button(new Rect (inv_x + inv_button_offset, inv_y + (c * inv_h), inv_w / 2, inv_h), "Drop")){
                                     inventory_script.rmv_from_inventory(current_item_to_display.id_name);
                                     update_inventory();
                                     //Rect windowRect = new Rect(80, 80, 120, 50);
                                     //windowRect = GUI.Window(0, windowRect, DoMyWindow, "My Window");
                                 }
                                 
                                 // next    
                                 if(current_item_to_display.next != null)
                                 {
                                     current_item_to_display = current_item_to_display.next;
                                 } 
                             } // end for