Comment 52 for bug 847001

Might not be related, but check if you have a high file descriptor limit configured (1048576). In my case that caused a delay in the pkexec cmd used to change the brightness. See https://forum.manjaro.org/t/very-slow-response-of-brightness-controls-in-gnome/74891/18 and https://gitlab.freedesktop.org/polkit/polkit/issues/87