As above, spstimings is a hint to the decoder as to the framerate, using the actual timestamps is far more accurate.
Slices can only reduce latency in some circumstances.
It won't give a big gain if decoding via any hardware acceleration (eg a Pi) as you won't have a second hw block to decode the second slice in parallel, in fact it may increase decode time as it requires extra setup.
Reduction in encode latency for the first slice is likely to be a couple of milliseconds out of ~40ms for a 1080p frame - it is not going to be huge.
The one place you may gain is over a low bandwidth link, where you can start sending the data a couple of ms earlier, and get the decoder started a similar few ms sooner.
IIRC it was added originally for a project where the input frame was also produced and fed to the encoder in slices, at which point you could save a bit more time. That option is not available on the Pi.
Tbh neither option is adds a huge amount for 99.9% of use cases, hence why they haven't been worried about before.
Software Engineer at Raspberry Pi Trading. Views expressed are still personal views.
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