How to remove xfce-panel popup/popdown delay if select autohide allways?

Based on https://forum.xfce.org/viewtopic.php?id=14676 create a new file of ~/.config/gtk-3.0/gtk.css and add the following lines to it.

#XfcePanelWindow {
   -XfcePanelWindow-popup-delay: 0;
   -XfcePanelWindow-popdown-delay: 0;
}

with above if you want to add a delay just remember that it is done in milliseconds, so if you want a 2 second delay the number would be 2000.

Hope this helps!