My intuitions want to say the point of diminishing returns is around the limits of human cognition. Stocks are ownership of some human activity, so once you start valuing them faster than human activity you're really redefining what stocks are. Now stocks become predictions on ownership of some human activity.

Since the first formalization of Sport (some might argue earlier), it's been a human tendency to go Faster, Stronger, Better. Moving from ownership of some human activity to predictions on ownership of some human activity is a manifestation of this Faster, Stronger, Better (since the prediction is itself a human activity... we're owning the human activity of predicting human activity).

All that is not to say any of this is inherently good or bad (I disagree at the loaded term 'rigged' being used by Lewis). There's no single cut-off point in additional value earned. It's more of a transition zone where you're not sure when exactly you left it, but at some point you realize on the other side is not the world you started in.