List applies the selections made the Display Options form to the data selected and then populates the Selected Data table with that formatted data. This formatting includes converting reporting units, providing Value and Flag and other features in the table. Depending on the amount of data selected, this can take time to prepare and display.
Quick List provides a basic listing of the selected data WITHOUT applying any formatting and shows the reporting units as they are stored in the Analyses table without making any unit conversions. Because Quick List does not perform any formatting, it is much faster than List and is valuable for viewing raw data.