Short simple code. Weird behaviour - ios

Question

I'm new to gcd and multithreading / concurrency / parallelism. I wrote this example project to test some things out. I have 3 methods, a list of image urls, and an imageView in the interface.
Each method does essentially the same thing, with minor variations.
loadAllImagesSlow() loops through the image urls, creating the NSData from each url, creating a UIImage and setting the imageView's image to that image.
loadAllImagesFast() dispatches that entire task to a global concurrent queue.
newMethod() loops through the image urls and dispatches each task to a global concurrent queue. This was the fastest! :D Exciting stuff. For me anyway.
When I run loadAllImagesFast or newMethod, I see the interface quickly cycle through each image, ending at the last one. But for loadAllImagesSlow, I don't see any images; only the last image appears at the very end.
I believe loadAllImagesSlow is running on the main thread (serial queue), because i don't get responses when I tap the screen (the other functions don't block the UI). But since it's a serial queue, I don't understand why the imageView doesn't display the images one at a time. Since the main queue is serial, everything's supposed to be executed before following code. But that isn't happening with the imageView's image being set.
Any ideas why that might be? I don't need this for a particular project; I'm simply trying to understand the behaviour.
class ViewController: UIViewController {
#IBOutlet weak var imageView: UIImageView!
var urlArray = [
"https://img0.ndsstatic.com/wallpapers/76df262f429c799124461286c5ee64b1_large.jpeg",
"https://www.walldevil.com/wallpapers/a74/olivia-wilde-women-actresses-green-eyes-ponytails.jpg",
"https://richestcelebrities.org/wp-content/uploads/2014/11/Olivia-Wilde-Net-Worth.jpg",
"https://cdn.pcwallart.com/images/olivia-wilde-wallpaper-3.jpg",
"https://kingofwallpapers.com/olivia-wilde/olivia-wilde-006.jpg",
"https://img0.ndsstatic.com/wallpapers/b39ca6a58368a76a92d56739d6e2da31_large.jpeg"
]
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
let tapGesture = UITapGestureRecognizer(target: self, action: #selector(self.handleTap))
self.view.addGestureRecognizer(tapGesture)
print("starting")
self.loadAllImagesSlow()
}
func handleTap(sender: UITapGestureRecognizer) {
print("tap")
}
func loadAllImagesSlow() { // takes around 4 seconds. UI is unresponsive throughout. Only last image gets shown
let startTime = Date()
for each in self.urlArray {
let url = URL(string: each)
do {
let data = try Data(contentsOf: url!)
self.imageView.image = UIImage(data: data)
} catch {
print("error")
}
}
let endTime = Date()
let executionTime = endTime.timeIntervalSince(startTime)
print("Execution time: \(executionTime)")
}
func loadAllImagesFast() { // takes around 4 seconds, but UI is responsive! Cycles through the images!
DispatchQueue.global(qos: .userInitiated).async {
let startTime = Date()
for each in self.urlArray {
let url = URL(string: each)
do {
let data = try Data(contentsOf: url!)
DispatchQueue.main.async {
self.imageView.image = UIImage(data: data)
}
} catch {
print("error")
}
}
let endTime = Date()
let executionTime = endTime.timeIntervalSince(startTime)
print("Execution time: \(executionTime)")
}
}
func newMethod() { // much much faster than 'loadAllImagesFast'
// This method harnesses the power of the concurrent queue by dispatching lots of smaller tasks that run concurrently, rather than dispatching one large task that runs concorruntly alongside nothing else in that queue; essentially, sequientially, on a seaprate queue.
for each in self.urlArray {
DispatchQueue.global(qos: .userInitiated).async {
let url = URL(string: each)
do {
let data = try Data(contentsOf: url!)
DispatchQueue.main.async {
self.imageView.image = UIImage(data: data)
}
} catch {
print("error")
}
}
}
}
}