When adding a large number of items to the collection, you should use the BeginUpdate method to prevent the collection from updating its internal state each time an item is added to the list. Once you have completed the task of adding items to the list, call the RibbonItemCollectionBase.EndUpdate method to resume regular processing.
This way of adding items dramatically improves performance and is recommended when adding more than 10 or 20 items to the list.
The code belows populates a RibbonComboBox with items containing the names of all the fonts installed in the system.
You can run the sample as-is, then comment out the calls to BeginUpdate and EndUpdate to see the difference in performance.