Magento loads a customer grid on making a new order from the admin section. If you are creating a new order from an existing order then the customer is already known and the grid should not be shown then.
This is running in production with various shops.
One of our customers complained that creating a new order from an existing order in the admin section of a Magento 1 shop was taking a considerable amount of time. We're talking more than 30s.
This module rewrites sales_order_create_customer_grid to Gracious_Gridoptim_Block_Adminhtml_Sales_Order_Create_Customer_Grid and breaks out of the __construct when a customer and it's id can be retrieved using Mage::getSingleton('adminhtml/session_quote')->getCustomer();
Load times improved by over 30 seconds for the customer's shop which has >720K unique customers in the customer_entity table. The page now loads within seconds.