I often miss the ability to compress uploaded videos and audio, because my clients don't know much about this. I assume this is not something you want to add directly in Prismic, but maybe you could consider a collaboration with someone like Transloadit or Cloudinary, so something could be marked for compression, or at least the developer could force media types to be compressed. Also serving these media files from a CDN would be great.
Issue that it solves:
Better client experience, no need for external compression tools. Faster websites.
I understand - but end users often don't know how to compress videos. And I can rarely use Youtube etc. for these custom websites. My clients use videos for fullscreen backgrounds, project thumbnails etc. so adding Youtube/Vimeo etc. is just not an option.