When you click on a COMBOBOX to view the list of items to select from, that is fast. The reason it is fast is because the items have already been sent from the back-end to the GUI client. There is no database activity or values being sent across the network at this time because it has already happened.
Nothing is free and the price in this instance is that any database activity to determine the items available and network activity to send the list items to the front-end takes place on or before when the window/form is visible.
If you have a COMBOBOX with hundreds or thousands of entries to select from then that can lead to excessive and unnecessary database and network activity. You should have a design standard to limit the number of entries in a COMBOBOX.
Read more at
https://4js.com/ask-reuben/ig-106/