There is significant latency when a user has many images in a list or dropdown list, particularly those with sub-choices that also have images. Because of this, we tried implementing design-time templates to allow lazy loading of images for ListBox and ComboBox. While implementing a solution is possible, it was much more complicated than expected.
One specific problem we encountered was that when using a template, the ListBox wraps its internal label in a span (which the ComboBox doesn't do). This results in problems with data-binding (requiring an item-by-item solution) and requires specialized css to handle. Even after the basic solution was working, sub-choices still had a problem accessing text and image data. This could be solved with a custom frontend runtime template, but it was not an elegant solution.
Our feature request is for lazy loading to be an option for images for the ListBox and ComboBox. This would significantly improve the user experience with long lists and drop-downs.