Whenever you use a Drop-Down List control and connect it to a data source by way of smartobject note that it will pull in all relevant data. Is if you connect it to a data cource with five thousand items all of them will be rendered by the control at runtime. That can lead to a very sluggish and slow loading form.
The Drop-Down List control is great when you want to serve up a small list but if we needed to serve up access to a larger list the Look-Up control is the prefered method as it will perform a search of the data source rather than rendering the whole thing and this leads to a more seamless user experience.