I think it would have to add a bit of latency, but I doubt it would be anything you could notice, but in a world where we are trying to get the latency from user input to display output as low as possible though, milliseconds would and do matter in the gaming hardware market from a marketing point of view. I don't know if you are aware but AOC (Active Optical Cables) with display port connectors do exist, they are typically for running very long runs, say if you want a 4K signal 100M away then that's how you could do it., I don't know too much about these as I haven't researched the tech but as they are AOC's the displayport plug on it will essentially be akin to an SFP on an optical network cable. I don't know if you know much about enterprise networking and storage solutions but it'll be pretty much the same and the electrical signals from the displayport out on the GPU and Input on the display will just do as they do now, no change and translation to light and back to electrical impulses will be handled on the cable connectors either end, that's why we call it an Active Optical Cable. I can't see them being beneficial on the desktop really at normal distances, but, for example, if you wanted to wire up a a big hall with large 4K displays for an Esports event ...well you could use Displayport over AOC's to get the signal out to the displays.
