Update README.md

examples/whisper.objc/README.md

@@ -19,3 +19,8 @@ open whisper.cpp/examples/whisper.objc/whisper.objc.xcodeproj/
 Make sure to build the project in `Release`:
 
 <img width="947" alt="image" src="https://user-images.githubusercontent.com/1991296/197382607-9e1e6d1b-79fa-496f-9d16-b71dc1535701.png">
+
+Also, don't forget to add the `-DGGML_USE_ACCELERATE` compiler flag in Build Phases.
+This can significantly improve the performance of the transcription:
+
+<img width="1072" alt="image" src="https://user-images.githubusercontent.com/1991296/208511239-8d7cdbd1-aa48-41b5-becd-ca288d53cc07.png">