The ComboBox control includes the EnableItemCaching property, which you can only us in load-on-demand scenarios.
By default, EnableItemCaching is set to False. When it is set to True, the combobox caches the returned items for each request.
This means that if you type a certain text and the combobox returns items, the items are cached, and the next time you type the same text, the server-side event ItemsRequested does not fire, and the items load from the cache.
This decreases the server round-trips, and improves server performance in high-volume instances.