Hello Stefan,
Just to understand the question right; you are using DMX to switch lights on and off and are wondering on the latency from the memoent the CueCore receives a trigger until the actual result arrives at the fixture, correct?
As you wonder about the cable length, it will definitely increase the overall latency, but I think, as long as you stay within the DMX specs, this increase will be within 1 ms and is not noticeable. The latency, from receiving to sending, within the CueCore is max 25 ms (40 fps), assuming its keeping up with the processing load it has to handle. If a CueCore has many things to do at the same time it might have trouble keeping up and then the latency might be longer.
I hope that this answers your question,
Best regards,
Jurrien