PHImageRequestOptionsDeliveryMode.fastFormat
Photos provides only a fast-loading image, possibly sacrificing image quality.
Declaration
case fastFormatDiscussion
Photos calls your resultHandler block once. If a high-quality image cannot be loaded quickly, the result handler provides a low-quality image. Check the PHImageResultIsDegradedKey key in the info dictionary to determine the quality of image provided to the result handler.
This option is available only if the isSynchronous property is false.