Ios 为什么PHPhotoLibrary performChanges creationRequestForAssetFromVideoAtFileURL如此缓慢？

根据视频的大小，我测量的时间从几十秒到几分钟不等。这个更改请求不应该很快（本地闪存盘拷贝/元数据/校验和），因为它会将本地视频添加到photolibrary/相机卷中吗当iCloud备份打开时，似乎会出现此问题。我一直无法通过web搜索或apple dev文档找到很多有用的信息

示例代码：

    func exportAssetToPhotoLibrary(videoURL: NSURL, _ exportedAsset: (localIdentifier: String) -> Void) {
        var localIdentifier = ""
        var startTime = NSDate.timeIntervalSinceReferenceDate()
        PHPhotoLibrary.sharedPhotoLibrary().performChanges({
            let assetRequest = PHAssetChangeRequest.creationRequestForAssetFromVideoAtFileURL(videoURL)
            let assetPlaceholder = assetRequest.placeholderForCreatedAsset
            localIdentifier = assetPlaceholder.localIdentifier
        },
        completionHandler: { success, error in
                var elapsedTime: NSTimeInterval = NSDate.timeIntervalSinceReferenceDate() - startTime
                NSLog("exported cameo video to photo library \(elapsedTime)")
                exportedAsset(localIdentifier: localIdentifier)
        })
    }