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Very short segment durations are effective only when latency is more important than quality.

Each TS segment must start with a key-frame, and the GOP size can't exceed the duration of a segment (e.g. one second). Lowering the segment duration increases the frequency of key-frames, which has the effect of lowering the quality you can achieve at a given bitrate.

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Note that this is a Apple requirement for HLS. Most people don't realize that the GOP size doesn't impact the latency, but it impacts start-up time.