Create a decompressed UIImage explicitly so we're able to do it on a non-UI thread. Typically UIImage loaded from a file is decompressed into an actual bitmap lazily upon use.

UIImage+Decompressed.swift

extension UIImage {
    func decompressed() -> UIImage? {
        guard let image = self.cgImage else { return nil }
        let colorSpace = CGColorSpaceCreateDeviceRGB()

        guard let context = CGContext(data: nil, width: image.width, height: image.height, bitsPerComponent: 8, bytesPerRow: image.width * 4, space: colorSpace, bitmapInfo: UInt32(CGImageAlphaInfo.premultipliedFirst.rawValue | CGBitmapInfo.byteOrder32Little.rawValue)) else { return nil }

        let rect = CGRect(origin: .zero, size: CGSize(width: image.width, height: image.height))
        context.draw(image, in: rect)
        let decompressedImage = UIImage(cgImage: image, scale: UIScreen.main.scale, orientation: .up)

        return decompressedImage
    }
}