Twitch latency in low-latency mode is ~2s (you can check for yourself in the settings cog menu -> advanced -> video stats) because of "prefetch segments", which are delivered via HTTP 1.1 streaming body responses. The client requests this segment and long polls while frames are delivered as they're being encoded. It's simpler and cheaper than WebRTC or Apple's low-latency HLS parts, and scales to hundreds of thousands of viewers. The downside is that it's not part of the HLS specification so support for it needs to be bespoke, but it is a proven technology that wasn't covered here (and to my knowledge, the most widely used low latency HTTP solution by # of users). Twitch has been using this for years.
The reason I didn't talk about it in the post is because the basic ideas that go into reducing the latency of any HLS/DASH implementation are the same. Smaller segment sizes, chunked transport, and smart encoding and prefetch/buffering implementations.
But ... while there's lots of terrific engineering underpinning the Twitch approach, there's no way around the real-world limits to that approach. As a result, on good network connections you'll usually get ~2s latency. But not so much on the long tail of network connections. If your use case can gracefully accommodate a range of latencies for the same shared session across different clients, that's fine. If it can't, it's not fine. Plus, I don't think you can get down very much below 2s.
(I don't have access to any Twitch/IVS internal data, so I don't know what latencies they see globally across their user base. But I've done a lot of testing of this kind of stuff in general.)
Agreed, all of the early HTTP based streaming was about HTTP progressive download. Microsoft’s smooth streaming was dominant in the early 2000s used it too. Glad that Twitch has been using it.
For me, DASH and HLS are just manifests or playlist definitions, of how best to find the resource. It is not dissimilar to webrtc signalling, i.e., go to a resource to discover where to get other parts of the resource.
