Opening applications menu for the very first time is slow, it may take even several seconds. Subsequent openings of the menu are fast, which indicates that the data are cached somewhere. This click on applications menu button will be the first action many users will do after logging into the Xfce session. A proposal In addition to building a menu in a mouse-press event handler, this could also be done in an idle timer started at the time when the plugin is constructed. So that when the button is clicked the menu is already loaded.
It is affected for me too. Really it is uncomfortable that I must to wait for a time with the such simple action.
It affects two apps : xfce4-panel's application menu plugin (either via click on the plugin button, or xfce4-popup-applicationsmenu in a term/commandline), and xfdestkop when it's set to show the app menu on rightclick. In xfdesktop, the menu is created in src/xfce-desktop-menu.c, main entry point seems to be xfce_desktop_menu_new(). This one could be called with true to idle-load the menu, maybe directly from one of the xfdesktop constructors called at startup. see http://git.xfce.org/xfce/xfdesktop/tree/src/xfce-desktop-menu.c#n214 In the panel appmenu (see plugins/applicationsmenu/applicationsmenu.c), it's loaded in applications_menu_plugin_menu(), but if that function is called it calls inconditionally gtk_menu_popup at the end - maybe loading the menu should be separated from popping it, so that it can be loaded by, say applications_menu_plugin_construct() for example. see http://git.xfce.org/xfce/xfce4-panel/tree/plugins/applicationsmenu/applicationsmenu.c#n1070
Still happening on Ubuntu Studio 14.10. $ xfce4-panel --version xfce4-panel 4.11.1 (Xfce 4.10)
Affects me too. Also, it isn't just the first time I want to open the applications menu. Whenever I don't use it for some time, it gets slow again and takes over 5 seconds to appear.
Parts of xfdesktop and panel have been rewritten since this bugs. If the problerm is still present on 4.14, please reopen a bug on gitlab