Created attachment 8236 
Simple workaround for repeating xfce_shortcuts_grabber_keys_changed calls

Whenever I connect an external USB keyboard, xfsettingsd's and Xorg's CPU usage increases. In GNOME System Monitor xfsettingsd's Status changes from Sleeping to Running and it takes 1-2% CPU; Xorg's CPU usage jumps from 1-2% to 25%, i.e. the entire CPU core in my 4-core system. During this high CPU usage time (~15s) keyboard shortcuts - configured in XFCE settings -> Keyboard -> Application Shortcuts - don't work.

I have this problem for many months (maybe years) now. A similar shortcuts-not-working-for-a-time issue happens when resuming from hibernation. Similar issues were reported: https://forum.manjaro.org/t/keyboard-shortcuts-stop-working-after-reconnecting-the-usb-keyboard-when-xmodmap-is-present/39482, https://forum.manjaro.org/t/keyboard-shortucts-stop-working-in-xfce-after-user-switch-resume/35849/9 and on other forums/bug trackers as well. I don't use Xmodmap, but I do have 2 keyboard layouts configured. The issue happens on my Manjaro system with the packaged versions of XFCE packages as well as with the latest git versions of libxfce4ui, exo, xfconf and xfce4-settings. The issue does not happen on a Debian stable (stretch) system with its packaged XFCE versions with similar configuration (2 layouts).

When I attach to the xfsettingsd process in KDevelop and interrupt it right after plugging in a USB keyboard, I get xfce_shortcuts_grabber_keys_changed->...->xfce_shortcuts_grabber_grab in the frame stack. I made the following change in libxfce4ui's xfce-shortcuts-grabber.c:
-  TRACE ("Keys changed, regrabbing");
+  DBG ("Keys changed, regrabbing");
The line
"DBG[xfce-shortcuts-grabber.c:168] xfce_shortcuts_grabber_keys_changed(): Keys changed, regrabbing"
is repeated 50 times in xfsettingsd's output when I launch it like this: (XFSETTINGSD_DEBUG=1 xfsettingsd --no-daemon --replace &> /home/igor/Documents/tmp/xfsettingsd-git.log &). The same line is repeated 525 times in the output when I connect an external USB keyboard.

Any ideas why the GDK keys-changed signal is emitted so many times? If it is not possible to stop GDK from emitting the signal repeatedly, perhaps a workaround should be implemented in libxfce4ui that handles this signal at most once each 5 seconds.
For example, store the timestamp of the last call to xfce_shortcuts_grabber_keys_changed. If the next call happens before 5 seconds elapse, schedule executing xfce_shortcuts_grabber_keys_changed in 5 seconds time and return from it immediately. Then return immediately from the future calls to this function until the 5 seconds pass and xfce_shortcuts_grabber_keys_changed is called by the 5s-timer.

In fact, even a much simpler workaround - simply ignoring xfce_shortcuts_grabber_keys_changed calls for 5 seconds after its last full execution - works for me: the high CPU usage practically disappears, shortcuts work without interruptions. One downside of the simple workaround is that without knowing what exactly triggers repeated keys-changed signals, there is a risk of some configuration loss. Also if genuine configuration changes happen twice in the course of 5 seconds, the second one won't be handled. The simple workaround patch is attached to this issue for illustration and experimentation purposes.