Delphi’s TComboBox combines an edit box with a scrollable list – with the Style property defining the display style of the combo. When set to csDropDownList there’s no edit box – users can only pick from the predefined items displayed by the list. If you need to react, when the selection changes, you will write code for the OnSelect event.
Now, presume our combo is a kind of a filter selector – when the selection changes you need to update some other UI control. For example, combo lists invoices and selecting one (item from its list) displays the selected invoice items in some grid control.
Depending on the number of invoice items, where they are stored, how you fetch them, and so on – the actual code to (fetch items and) refresh the UI might take longer time – whatever longer means. If you scroll quickly through the combo items – OnSelect will get fired for each visited item in the list and your application might appear irresponsible, lagging and slow.