The standard Delphi TComboBox is based on WinAPI. Every item you add to the list triggers a WinAPI "SendMessage" call, which takes much (x100) longer than adding an item to a TStringList.
To overcome this delay, with the help of AI, I designed a slimmed down replica of TComboBox based on TStringList:
https://github.com/bLightZP/ZPComboBox
If you have a form with lots of TComboBox drop down list components and it's not opening fast enough for you, try this.
Limitations
The only event supported is OnChange.
The only style supported is csDropDownList.
The pop-up listbox is shown in a pop-up window, so the underlying window's title bar changes color when losing focus.
The styling is close to TComboBox, but not identical.