Delay between viewWillAppear and viewDidLoad after entering foreground

Question

A delay of 2 seconds occurs between viewWillAppear and viewDidAppear only in one screen when application comes back from the background. Normally, there isn't a delay in this screen when the app is active. In debugging, it seems both viewWillAppear and viewDidAppear are completed in a few milliseconds.

In the screen, there is a horizontal scrollview of 9 pages and in each one, there is a tableview. I realized that the delay disappears when the below code of tableviews' reload is removed from viewWillAppear.

for i in 0 ..< self.levelTables.count {
   self.levelTables[i].tableView.reloadData()
}

I don't understand why this causes delay. When the app is active, there is no delay. It only appears when the application comes from the background. What's the difference? If it takes time, why is this delay not occurring always?

Any ideas?