iOS Collection View Lagging - swift

Question

In my App I am using a CollectionView to display some Images. When I start scrolling performance is quite poor and big lags occur. I tried utilising an Image cache so the Images are not reloaded every time the cell appears. I have also tried removing the rounded edges and utilising layer.shouldRasterize. Nothing seems to help. The Images are in the Assets of the App.
let imageCache = NSCache<AnyObject, AnyObject>()
...
class MainMenuController: UIViewController, UICollectionViewDataSource, UICollectionViewDataSourcePrefetching, UICollectionViewDelegateFlowLayout {
#IBOutlet weak var collectionView: UICollectionView!
let reuseIdentifier = "cell"
override func viewDidLoad() {
super.viewDidLoad()
....
}
//Collection View
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return items.count
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath as IndexPath) as! MainMenuCell
cell.titleLabel.text = self.items[indexPath.item].getName()
cell.titleLabel.textColor = ColorInterface.sharedInstance.ultra_textcolor
if let imageFromCache = imageCache.object(forKey: items[indexPath.item].getKey() as AnyObject) as? UIImage {
cell.imageView.image = imageFromCache
}
else
{
let imageToCache = UIImage(named: items[indexPath.item].getImage())!
imageCache.setObject(imageToCache, forKey: items[indexPath.item].getKey() as AnyObject)
cell.imageView.image = imageToCache
}
cell.layer.cornerRadius = 7
return cell
}
func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
let totalHeight: CGFloat = (collectionView.frame.width / 2)
let totalWidth: CGFloat = (collectionView.frame.width / 2)
return CGSize(width: ceil(totalWidth - 5),height: ceil(totalHeight - 5))
}
func collectionView(_ collectionView: UICollectionView, prefetchItemsAt indexPaths: [IndexPath]) {
for index in indexPaths
{
if imageCache.object(forKey: items[index.item].getKey() as AnyObject) == nil
{
let imageToCache = UIImage(named: items[index.item].getImage())!
imageCache.setObject(imageToCache, forKey: items[index.item].getKey() as AnyObject)
}
}
}
}
After I scrolled to the bottom once everything is silky smooth which originally lead me to believing it is the loading of the images which is causing problems. What am I doing wrong here ?