insertSubVIew taking a "long" time - swift

Question

In my container controller, the user can pan the views to switch to different views. When the pan gesture begins, it's add the view of the new view controller to the view with: view.insertSubView(view:, atIndex:)
After researching a bit, I noticed this step takes about 0.03 sec (while the other things are all 0.001-0.002 sec). This causes the transition to stotter a bit which is kind of annoying.
The view controller is created at the beginning of the app as a global, using the storyboard.
Also, this only happens when the view is loaded for the first time. The transitions are all fluently after.
What can I do to preload the views so it won't take so "long" when its loaded for the first time?
EDIT:
SURROUNDING CONTEXT:
var pendingViewController: UIViewController! {
didSet {
if let pending = pendingViewController {
addChildViewController(pending)
let index = view.subviews.count - 1
NSLog("start insert view test")
view.insertSubview(pending.view, atIndex: index)
NSLog("end insert view test")
pending.didMoveToParentViewController(self)
}
}
}
Because it only happens when the view is loaded for the first time I was thinking the problem could be somewhere with the viewDidLoad and viewWillAppear. The results are shown below. Only viewDidLoad took a small amount of time (0.005 seconds). There's a gap of 0.02 sec before getting to viewDidLoad though, but I have no idea what it could be.
2015-12-17 15:15:57.116 Valor[777:232799] start insertView view test
2015-12-17 15:15:57.136 Valor[777:232799] start viewDidLoad test
2015-12-17 15:15:57.141 Valor[777:232799] end viewDidLoad test
2015-12-17 15:15:57.142 Valor[777:232799] start viewWillAppear test
2015-12-17 15:15:57.144 Valor[777:232799] end viewWillAppear test
2015-12-17 15:15:57.146 Valor[777:232799] end insertView view test

Answer 1

Use instruments to find out where slow code is happening, not log statements with timestamps. This will show you (including system calls) exactly where the time is being spent.
Inserting subviews can be slow because of layout. However, your trace (such as it is) suggests the time is being spent creating and loading the view of the view controller. You say this view comes from a storyboard. What is in there? How many other things get triggered when this view loads? Use the time profiler and you will be able to tell. It could be something as simple as a property you're giving a default value to that could instead be a lazy value.
The view controller is created at the beginning of the app as a global, using the storyboard.
If this is the case then you can force loading of the view controller's view by doing something like
let hack = viewcontroller.view
Accessing the view property of the view controller causes it to load up the view from the storyboard.

Answer 2

At first sight, I see one strange thing: the index for inserting pending.view. If the view count on your view is 5 for example, that means that the indices of its subviews vary from 0 to 4. And then, you want to put the new subview at index 4 (= 5 - 1) while there's one already on your view which has index = 4. AFAICS, you meant 5 instead of 4. So, you should ether stick to let index = view.subviews.count and then insert the new view at that index, or just use view.addSubview(pending.view). I hope this will help you out.