DNG files are more complex than JPEG files, as they contain more data and higher-quality information for editing, which can result in longer processing times. The speed can also depend on the capabilities of your machine; larger and more complex files like KNGs require more processing power, which may slow down transfer times.
You should either edit your original RAW/DNG files with Photo AI as a standalone application and then import them to Apple Photos for further edits, otherwise you should convert your files before using the Apple Photos plugin as it will always export a JPEG file.