Reduce color bandings to create GIF from at most 200 video frames using Swift/Objective-C

Question

I want to make GIF from video frames. Per frame (UIImage) size is at most 960 * 540. As GIF is bound to 256 colors, I got more colors issues. frames are poorly generated. After so many research, I've found Dithering approach that may reduce color bandings.
I didn't go with this approach because default time complexity of dithering is O(N^2) where N is the multiplication of Image's width and height. I don't know how to reduce color banding efficiently without dithering for 200 frames. Is there any other technique or filter available which can reduce color banding efficiently?
Original Video : Original video
The GIF I produced : my GIF
The GIF I need : better-GIF

My Approach: First I saved all frames as UIImage from Video. then I created GIF using following code :

func generateImagesToGif(photos: [UIImage], filename: String) -> Bool {
        let documentsDirectoryPath = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)[0]
        let path = documentsDirectoryPath.appending(filename)
        let fileProperties = [kCGImagePropertyGIFDictionary as String: [kCGImagePropertyGIFLoopCount as String: 0]]
        let gifProperties = [kCGImagePropertyGIFDictionary as String: [kCGImagePropertyGIFDelayTime as String: 0.5]]
        let cfURL = URL(fileURLWithPath: path) as CFURL
        if let destination = CGImageDestinationCreateWithURL(cfURL, kUTTypeGIF, photos.count, nil) {
            CGImageDestinationSetProperties(destination, fileProperties as CFDictionary?)
            for photo in photos {
                CGImageDestinationAddImage(destination, photo.cgImage!, gifProperties as CFDictionary?)
            }
            return CGImageDestinationFinalize(destination)
        }
        return false
    }

Update : Have a look this (High Quality GIF). Their GIF looks like original video. Very low color bandings!