What do you want to do with the Decklink input? Do you only want to add graphics to the video? That is called internal keying and there is a discussion about doing this on a Decklink Duo 2 here. This gives you no latency to the incoming video. If you would want to manipulate the incoming video, like scaling it etc. then you would need the play command. That gives you about 7 frames of latency.