For the last couple of years, it has taken a long time (sometimes as long as 30 seconds!) for the list of available WiFi networks to be displayed after clicking the icon at the top of the screen.
I have over 400 "preferred" (saved) networks. Does that have something to do with it? I like to keep them because I travel a lot and it's nice to come back to a place and have my system automatically log in without any effort on my part.
I have written a command line application to manage WiFi independently of the GUI (at https://github.com/keithrbennett/wifiwand), but would prefer to have both options available to me.
Best Answer
Having over 400(!) preferred networks may be one symptom of the lengthy time it takes to load them all. I suggest that you take the time to delete the networks you never use, even ones that you connect to infrequently. It may be convenient to be able to connect quickly, but you've found one reason why it actually isn't. Re-establishing networks does not take much time, and you may just prevent your computer from connecting to infrequently connected networks which have been compromised since you last used them.
Secondly, if you use iCloud Keychain, any Wi-Fi networks you connect to on any device you use with iCloud Keychain (iOS devices particularly) will populate this list. I found this out when coming back from a vacation in Europe and all the networks I connected to on my iPad whilst there appeared on my desktop Mac at home!
Lastly, it is wise to move the most frequently used Wi-Fi networks to the top of the list. This may help decrease the amount of time the list appears. I have but a fraction of the number of networks saved, so I can't do a real-world experiment to see if this will decrease the length of time it takes to load the list.