NSCoding UIImage to disk takes up huge amount of disk space and writes slowly

Question

I am serializing UIImages to disk using NSKeyedArchiver.

UIImage conforms to NSCoding so serializes+deserializes properly but the resulting file is quite large causing performance issues.

I tested this by loading images from the ALAssetsLibrary taken on a standard iPhone5 camera to a UIImage and then serializing images to disk

UIImage *testImage = [asset defaultRepresentation]....
[NSKeyedArchiver archive:testImage ....]

The files were ~15MB and took over 1 second to write/load.

Then I tried serializing the UIImage by converting to NSData:

NSData *imageData = UIImageJPEGRepresentation(UIImage *image, 0....1); // tried most compression all the way to least compression
[NSKeyedArchiver archive:imageData....]

The resulting files were magnitude smaller (so faster to read and write)

What is the suggested way to save images? are the underlying images from the ALAssetsLibrary also taking 15MB on disk to store the images?

Should I be using PNG representation?