The slow Wi-Fi'er spoils the broth: I've been reluctant to link to any news covering this story, because I think the journalists writing about it don't understand that it's well known that a slow user on an Wi-Fi network can drag the speed down.
The French paper is a formal analysis of the problem with lots of detail, and that's terrific. But it's a long-known phenomenon that users who connect at slower speeds occupy more time slots, reducing the overall speed of the network.
Some network administrators turn off backward compatibility for this reason, forgoing dropdown speeds to trade off better overall throughput.