viewDidAppear is delayed by 0.5 sec

We're doing something similar to this in my current project, but we decide which controller to show first in the AppDelegate.

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

    var storyboard: UIStoryboard!
    if isLoggedIn {
        storyboard = UIStoryboard(name: "main", bundle: nil)            
    }
    else {
        storyboard = UIStoryboard(name: "login", bundle: nil)
    }

    let viewController = storyboard.instantiateInitialViewController()!
    viewController.loadViewIfNeeded()
    window.rootViewController = viewController
}

This has the advantage that when the app loads it immediately take you to the correct ViewController without showing the wrong one briefly first.

Move the call to performSegue(withIdentifier:sender:) to viewWillAppear(_:) instead of viewDidAppear(_:) in case usersignedin = true.

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)

    if userDefault.bool(forKey: "usersignedin") {
        performSegue(withIdentifier: "login", sender: self)
    }
}

Also, the response of signIn method is not received on the main thread. Since you're not updating the UI on the main thread, that's why it is resulting in the delay.

Embed the UI part in closure of signIn method in DispatchQueue.main.async

Auth.auth().signIn(withEmail: loginEmail.text!, password: loginPassword.text!) { (user, error) in
    if user != nil {
        self.userDefault.set(true, forKey: "usersignedin")
        self.userDefault.synchronize()
        DispatchQueue.main.async { //here..........
            self.performSegue(withIdentifier: "login", sender: self)
        }
    }
    else {
        let alert = UIAlertController(title: "Invalid Email or Password", message: nil, preferredStyle: .alert)
        let okButton = UIAlertAction(title: "Ok", style: .default, handler: nil)
        alert.addAction(okButton)
        DispatchQueue.main.async { //here..........
            self.present(alert, animated: true, completion: nil)
        }
    }
}

This is correct system behavior. ViewDidLoad will always be called much sooner than ViewDidAppear. IF you want to change the base view controller based on userDefault.bool(forKey: "usersignedin"), add that logic to where you set the main view controller and swap them if already signed in.

Change the order of view controllers. Always load your UITabBarController first (as initial controller). Then in your UITabBarController viewDidLoad, check if the user is logged-in or not. If not, then show the login screen. This way your UITabBarController is always loaded first and then based on auto-logn condition, your login screen is displayed. So when the user is auto looged-in then your delay issue will not come.

I have used this concept in all of my apps and it works fine.

viewDidAppear(_:) happens right after the view appears. I suggest moving your code from viewDidAppear(_:) to viewWillAppear(_:).

you can also add your login code in the viewDidLoad() because this method is called even before viewWillAppear(_:).

in your case your code will be:

override func viewDidLoad() {
    super.viewDidLoad()

    if userDefault.bool(forKey: "usersignedin") {
        performSegue(withIdentifier: "login", sender: self)
    }
}

More info:

when a view is loading, there are different methods which get called in a specific order:

1. viewDidLoad() (once and only when the object is being created.)
2. viewWillAppear(_:)
3. [View appears on the screen]
4. viewDidAppear(_:)

when you push another view controller over this one, this view controller won't be removed from the view controllers' stack. the important thing is when you come back to this view controller, this time viewDidLoad() won't get called. so only these methods get called again in this order:

1. viewWillAppear(_:)
2. [View appears on the screen]
3. viewDidAppear(_:)

Best way is to decide in AppDelegate which controller to load based on Login Status

        let navigationController = UINavigationController()
        navigationController.navigationBar.barStyle = .black
        let storyboard = UIStoryboard(name: "Main", bundle: nil)

        if UserDefaults.standard.object(forKey: "Token") != nil{
            let initialViewController = storyboard.instantiateViewController(withIdentifier: "HomeViewController")
            navigationController.viewControllers = [initialViewController]
        } else{
            let initialViewController = storyboard.instantiateViewController(withIdentifier: "LoginViewController")
            navigationController.viewControllers = [initialViewController]
        }

        navigationController.navigationBar.barTintColor = UIColor.black
        navigationController.navigationBar.tintColor = UIColor.white
        navigationController.navigationBar.titleTextAttributes = [NSAttributedString.Key.foregroundColor : UIColor.white]
        self.window?.rootViewController = navigationController
        self.window?.makeKeyAndVisible()