I've experienced the same issue people have been posting about recently. After a Windows Update, the StartIsBack start menu has a delay when opening. I tried a lot of different things but eventually figured out there's a registry key that causes it. I believe it's related to News and Interests as @TehCactus said, and I think it's probably an old setting that has been unused for years if you've had News and Interests disabled (like me) but for some reason it's causing trouble now. If your install is newer or you use the News and Interests feature you probably don't have the delay bug.
Go to \HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Feeds and find EnShellFeedsTaskbarViewMode. Deleting this key fixed the issue for me instantly. I hope it works for everyone else too. You don't need to kill SearchApp.
