What is the most memory-efficient way of downscaling images on iOS?

In background thread, my application needs to read images from disk, downscale them to the size of screen (1024x768 or 2048x1536) and save them back to disk. Original images are mostly from the Camera Roll but some of them may have larger sizes (e.g. 3000x3000).

Later, in a different thread, these images will frequently get downscaled to different sizes around 500x500 and saved to the disk again.

This leads me to wonder: what is the most efficient way to do this in iOS, performance and memory-wise? I have used two different APIs:

• using CGImageSource and CGImageSourceCreateThumbnailAtIndex from ImageIO;
• drawing to CGBitmapContext and saving results to disk with CGImageDestination.

Both worked for me but I'm wondering if they have any difference in performance and memory usage. And if there are better options, of course.