My ISP upload is sometimes saturated this lockdown, even when we are all streaming identical versions of the same movie simultaneously. It’s not even possible to pre-download/sync and use this version in the Watch Together session (and some clients understandably don’t support download/sync).
Implementations already exist in webrtc, where clients help upload to other clients, for instance p2p-media-loader (GitHub - Novage/p2p-media-loader: An open-source engine for P2P streaming of live and on demand video directly in a web browser HTML page). Jellyfin also added support last year (https://github.com/jellyfin/jellyfin/pull/3194). There are standalone solutions popping up too (GitHub - LucCADORET/comeover: The comeover website) but I’m a big fan of the Plex UI and would love to see this become native.
I also suspect ISPs will continue to cap residential upload for the foreseeable future to prevent hosting, and I already try my best to prevent transcoding during Watch Together since I run it on a shield, so I think P2P makes a lot of sense! A lot of my friends created accounts specifically for Watch Together, so while I realize it’s not a money making feature, it may help grow the platform.