title: enumerateObjects block main UI?
body: <p>I try to fetch all GIF in Photos in async block like this:</p>
  
  <pre><code>DispatchQueue.main.async {
    allImageFetchResult.enumerateObjects(options: .concurrent, using: {(asset, _, _) in
      if asset.isGIF {
        assetSet.insert(asset)
      }
    })
  }
  </code></pre>
  
  <p>But, it seems block user interface, the interface doesn't response to my touch actions until the block task is complete.</p>