Twitch has seen a large impact from this feature:
https://github.com/w3c/media-source/issues/175#issuecomment-1045018594

See also: https://github.com/mozilla/standards-positions/issues/547

I've started some prototyping related to this. The patches on this branch should do the plumbing to expose MediaSource on worker threads and such. What needs to happen now is to plumb this properly into HTMLMediaElement's selection/attach/detach logic. I hoping to get a chance to look at this in the next week (but it is best effort in my free time :)).

https://github.com/aosmond/gecko/tree/ao_mse_worker