Article in current Scientific American (first para and bullet points, rest is paywalled).

Podcast by the author (free).

The author, Douglas Fox, argues that there may be physical limits to how intelligent a brain made of neurons can become, limits that may not be very distant from where we are now.

He makes evolutionary arguments at a couple of points, suggesting that he is talking about how smart an organism could have evolved, rather than how smart we might make ourselves; he certainly isn't talking about how smart a machine we might create out of different materials.

From the podcast (I don't have access to the article):

Four routes to higher intelligence, which he argues won't get us very far:

1. Increase the speed of axons. But that means making them fatter, which drives the neurons further apart, neutralising the gain.
2. Increase brain size. That needs more energy, and before long you get something unsustainable. You get longer pathways in a larger brain, which slows them down. The neurons will make more connections, which makes them bigger, so the number of neurons scales slower than the volume of the brain. And anyway, whales and elephants have bigger brains than us but don't seem to be more intelligent, and cows have brains a hundred times the size of a mouse brain but aren't a hundred times smarter. So brain size doesn't seem to matter; at best the relationship with intelligence is unclear.
3. Packing more neurons into the existing volume by making them smaller. You run into signal to noise problems. The ion channels involved in generating action potentials are a certain size, and you must have fewer in a smaller neuron, hence more random variation. The result is neurons spontaneously firing.
4. Offload intelligence support. Books and the internet will remember things for you and help you tap the collective intelligence of your social network. Compare social insects doing things that they couldn't do individually. But by alleviating the necessity of intelligence this may even have reduced the evolutionary pressure to get smarter.

He's described simply as an "award-winning author", but I don't know if he has any scientific background, and there are too many people of the same name to Google him.