autostart will load as soon as the desktop start load. but many times that will cause problems like conky, will not have a menu when click on it. like tilda, will have fack transparent background. so if there is a delay before autostart apps load will be better. or some options to choose.
Absolutely not. If an app requires another app to be fully running, it needs to make sure of that on its own. Autostart doesn't provide a mechanism for ordering or priorities.
so why gnome provide such function? and also windows? all apps start later than desktop, i think this need reconsider. because apps added in autostart will start even before xfce4. and have a delay will not slow down the speed of xfce4, still can start them in parallel. maybe just let the xfce4 have a higher rundevel?