That's true. Most 2.4GHz networks beacon at 1Mbps, which is because they are typically configured for compatibility down to 802.11b and beacons are transmitted at the lowest enabled speed.
Here [1] somebody collected measurements of time eaten by beacons at few configurations. Not a complete disaster, but still somewhat significant. For example, at the place where I am now, I'm receiving 67 beacons per second (all at 1Mbps), which, according to those calculation, wastes 17% of airtime.
By just counting the literal airtime of the beacons, I think it underestimates the effects a bit, because it doesn't account for the contention of the remaining air time, which would be reflected in increased collisions and small delays which (sorry to be hand-wavy again) can add up. I think if he ran some application-level tests at the same time (perhaps iperf, perhaps something more sophisticated) he would see a bigger impact to "good-put".
Here [1] somebody collected measurements of time eaten by beacons at few configurations. Not a complete disaster, but still somewhat significant. For example, at the place where I am now, I'm receiving 67 beacons per second (all at 1Mbps), which, according to those calculation, wastes 17% of airtime.
[1] http://wifinigel.blogspot.com/2013/08/its-well-known-rule-of...