> My impression is that Twitch and YouTube livestreaming are using a 10-30 second delay relative to realtime,
Yeah. The rule of thumb with Twitch used to be 11 seconds. You can still measure this because many streams replay the chat in the stream as an overlay for both being able to see when the streamer has seen your message and for archival purposes to preserve the chat in VODs.
> don't understand how this interacts with the typical ability to start watching almost instantly, with no visible buffering delay, though.
There's a buffer on the CDN (which they have anyways because they're recording the VOD) and you start playback at the point t seconds back.
There is a low-latency mode for twitch now that I think is enabled by default. The actual delay is reported in the "stats" view under advanced settings, probably coming from timestamps in the video metadata. There's third-party twitch clients I've used with the option to delay the chat to match the video using this information, which would be useful if you didn't want chat popping off to spoil you.