Given a CIImage, what is the fastest way to write image data to disk?

I would suggest passing the CGImage directly to ImageIO using CGImageDestination. You can pass a dictionary to CGImageDestinationAddImage to indicate the compression quality, image orientation, etc.

CFDataRef save_cgimage_to_jpeg (CGImageRef image)
{
    CFMutableDataRef cfdata = CFDataCreateMutable(nil,0);
    CGImageDestinationRef dest = CGImageDestinationCreateWithData(data, CFSTR("public.jpeg"), 1, NULL);
    CGImageDestinationAddImage(dest, image, NULL);
    if(!CGImageDestinationFinalize(dest))
        ; // error
    CFRelease(dest);
    return cfdata
}

Try using the CIContext method writeJPEGRepresentation (available iOS 10) as follows to eschew UIImage and make the writing faster.

extension CIImage {

    @objc func saveJPEG(_ name:String, inDirectoryURL:URL? = nil, quality:CGFloat = 1.0) -> String? {
        
        var destinationURL = inDirectoryURL
        
        if destinationURL == nil {
            destinationURL = try? FileManager.default.url(for:.documentDirectory, in: .userDomainMask, appropriateFor: nil, create: true)
        }
        
        if var destinationURL = destinationURL {
            
            destinationURL = destinationURL.appendingPathComponent(name)
            
            if let colorSpace = CGColorSpace(name: CGColorSpace.sRGB) {
                
                do {

                    let context = CIContext()

                    try context.writeJPEGRepresentation(of: self, to: destinationURL, colorSpace: colorSpace, options: [kCGImageDestinationLossyCompressionQuality as CIImageRepresentationOption : quality])
                    
                    return destinationURL.path
                    
                } catch {
                    return nil
                }
            }
        }
        
        return nil
    }
}

The @objc keyword enables you to call the method in Objective-C as:

NSString* path = [image saveJPEG:@"image.jpg" inDirectoryURL:url quality:1.0];

For PNG there is a similar method writePNGRepresentation for iOS 11:

   if #available(iOS 11.0, *) {

        if let colorSpace = CGColorSpace(name: CGColorSpace.sRGB) {
                
            do {
                let format = CIFormat.RGBA8
                    
                try context.writePNGRepresentation(of: self, to: destinationURL, format: format, colorSpace: colorSpace)
                    
                return destinationURL.path
                    
            } catch {
                return nil
            }   
        }
    }