https://stackoverflow.com/questions/62802007/loading-multiple-images-from-document-directory-using-concurrent-queue-but-ui-i#comment111104362_62802007

ConcurrentImageLoad.swift

private func loadImagesConCurrent() {
    let concurrentQueue = DispatchQueue(label: "myConcurrentQueue", qos: .background, attributes: .concurrent)
    activity.startAnimating()
    let group = DispatchGroup()
    for (i, item) in myArray.enumerated() {
        group.enter()
        concurrentQueue.async { [weak self] in
            let fileURL = try! FileManager.default
                .url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: true)
                .appendingPathComponent(item as! String)

            guard let image = UIImage(contentsOfFile: fileURL.path) else { 
                group.leave()
                return
            }

            DispatchQueue.main.async {
                self?.imagesCollection[i].image = image
                print("loaded")
                group.leave()
            }
        }
    }
    group.notify(queue: .main) { [weak self] in 
        self?.activity.startAnimating()
    }
}