There's a significant delay in Ubuntu responding to volume up/down from my keyboard (about 1 - 3 seconds) in Ubuntu 20.04

Question

this one's got me a bit stumped. Note that this is on a fresh install of 20.04, not an upgrade.

The issue can be summarised as:

• At any time, if I attempt to use the keyboard's volume keys, there is a significant delay (between 2 and 5 seconds) before the volume actually changes, and before the volume indicator shows up on the screen.
• At any point after changing the volume in this manner, it takes Ubuntu about the same amount of time to respond to my first normal keyboard event (e.g. typing a letter), after which it behaves as normal.
• It's almost as if Ubuntu has to toggle between "volume state" and "normal input state".

I took the liberty of tailing syslog when changing volume, and then using the keyboard normally afterwards, and saw this output:

Apr 27 16:01:53 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 38 with keysym 38 (keycode 11).
Apr 27 16:01:53 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 32 with keysym 32 (keycode b).
Apr 27 16:01:53 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 31 with keysym 31 (keycode a).
Apr 27 16:01:53 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 33 with keysym 33 (keycode c).
Apr 27 16:01:53 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 39 with keysym 39 (keycode 12).
Apr 27 16:01:53 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 34 with keysym 34 (keycode d).
Apr 27 16:01:53 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 35 with keysym 35 (keycode e).
Apr 27 16:01:53 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 36 with keysym 36 (keycode f).
Apr 27 16:01:53 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 37 with keysym 37 (keycode 10).
Apr 27 16:02:12 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 38 with keysym 38 (keycode 11).
Apr 27 16:02:12 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 32 with keysym 32 (keycode b).
Apr 27 16:02:12 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 31 with keysym 31 (keycode a).
Apr 27 16:02:12 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 33 with keysym 33 (keycode c).
Apr 27 16:02:12 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 39 with keysym 39 (keycode 12).
Apr 27 16:02:12 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 34 with keysym 34 (keycode d).
Apr 27 16:02:12 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 35 with keysym 35 (keycode e).
Apr 27 16:02:12 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 36 with keysym 36 (keycode f).
Apr 27 16:02:12 StuartUbuntu gnome-shell[1743]: Window manager warning: Overwriting existing binding of keysym 37 with keysym 37 (keycode 10).