It is possible to keep persistent level almost empty. When game starts sub-levels which have type “AlwaysLoaded” will be loaded and visible before BeginPlay event. Sub-levels of a “Blueprint” type by default are not loaded, and should trigger loading on user events which is obviously after BeginPlay event. But you can make them loaded and visible at game startup as well, using “Edit Properties” command in “Levels” tool and setting “Initially Loaded” and “Initially Visible” to true.