Your router is a switch. Those are point-to-point and don’t go through a bottleneck of the “slowest” device.
Yes, I understand the difference between a switch and a hub. But this is a wireless network, not ethernet cables to a switch. Here is what the article says to do:
Another way to separate the client types is to use a dual-band, dual-radio N router. You would connect your G devices to the 2.4 GHz radio and your dual-band N devices to the 5 GHz radio. But this has the downside of shorter range for the 5 GHz band devices.
It is my understanding that some wireless routers do this automatically, for example Apple's AirPort Extreme.
Here is another article: How-to: set up dual-band WiFi (and juice your downloads)
The article also gives instructions to set up two wireless routers: one router at 5GHz for your faster devices using 802.11n and another router at 2.4GHz for your slower devices using 802.11g.