Takes Long Time to Show Network List After Clicking WiFi Icon
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.