There is an official download option inside the app. If they limit the download speed to the watching time, it won't be useful.

The official download option doesn't download it to your filesystem as a file. It just lets you watch the video offline in the official app/website. Just tested it now.

Meaning the video file exists in your file system somewhere, so downloading at a higher speed than possibly viewing the video is an existing functionality in the app.

If you're on iOS no way to access it, and I think on Android it's in protected storage as well.

This option is only available to premium users afaik