Macbook 2013 vs 80 MHz WiFi channels

I found that the WiFi on my MacBook Pro (late-2013) works slowly on 5 GHz / 80 MHz channels (topping out at about 30 Mbps). Enough so that it's at least 3x faster when connected to the 2.4 GHz band in the same router (and I say at least because I can't measure any higher).

So, if your router works in the 2 bands and your Mac is switching between them transparently (as it usually does, which usually is advantageous), then you would be seeing big, mysterious changes in speed.

The solution for me was to set the router to work on 40 MHz channels in the 5 GHz band; previously, it was set to Auto channel width, which makes it try using the 80 MHz width, and looks like the MacBook didn't like that.

One interesting detail is that both my iPhone 7 and iPad Pro seemed to work well with the 80 MHz channels; only the MacBook was slow, probably because of its older hardware. Once I forced 40 MHz, all the devices routinely get >100 Mbps. Of course setting 40 vs 80 MHz surely limits the theoretical maximum speed, but I wasn't going to reach it anyway in my usage.

Also note that the extra information that appears when Option-clicking the WiFi menu shows nothing to help diagnose this problem: my MCS, Tx Rate, PHY mode, RSSI, etc stayed the same once I changed the router's configuration. Particularly, the channel width was always shown as 40 MHz!

So the only hint you may get that this is actually your problem is that Wireless Diagnostics.app's Scan will show that the WiFi SSID to which you are connected is using 80 MHz, while the WiFi menu says that you're using 40 MHz.

Finally, it's also interesting to note that usually routers do a good job when in their default settings; only this particular interaction of 80 MHz vs 2013 Macbook is problematic in my router (Asus TM-AC1900 / RT-AC68U) and required me to take it out of the Auto width setting.

This also means that probably a lot of Macbooks out there have this hard-to-diagnose problem...

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