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in order to start rendering video, the clients need to receive an initial key frame. the key frame interval is set to 120 (frames) here, which depending on your frame rate could be those ~3s. ffmpeg has a

force_key_frame

option that you could try to use to force more key frames at the beginning of the file (this will also increase the bandwidth needed to transmit at the beginning of the file), but I have never tested this myself

Thanks, I'll give it a try. Will keep you posted!