How about a right click on the desktop? I have a very fast computer with no bloatware on, yet it takes half a second for the desktop context menu to appear. When I do this repeatedly. The first time takes 1 second or more.
Compare with a right click menu in a browser which is instant.
I can't test it on my current computer, but does the setting affect menus that are triggered by hovering over something?
If so, then 400ms makes more sense, and the real bug is that menus summoned by an explicit click should be exempt from the delay.
The reason for a hover-delay is that it allows someone to flick their mouse to their real destination without triggering a "trap" of content which pops up on the way to obscure their goals.
Compare with a right click menu in a browser which is instant.