After upgrading my xubuntu to 19.04 the use of my Brightness and Volume keys have a delay of 30+ sec and it looks similar to this topic: Fn Media keys slow/delayed on ubuntu gnome 17.04. I am using a HP Pavilion Laptop.
With ubuntu it works.
I think it is about the small pop-up message (Gnome's on-screen-display?), cause it does not pop up and every other fn-key not displaying a notification works flawlessly.