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.