Apple - Mac Wifi Slow, Fine on Other Devices

After speaking to a friend I established that this problem is potentially due to the macOS Sierra update. I followed the first step in this link - removing existing Wi-Fi preferences. Which seems to have worked for now.

I had the exact same issue. I went through every link on every result from Google by others who had this issue and nothing worked for me.

A user in one of the Apple threads suggested creating a new user profile, because that had worked for them. When I created a new profile, I got an incessant popup that asked me to login to a VPN account (I have NordVPN but it wasn't running at the time and I barely use it) -- this popup was blocking my access to my wifi though! So I rebooted in safe mode and logged into the new profile. Sure enough, the wifi was back to being fast.

I looked up info about the popup and apparently it's another known issue on Macs, and the only solution is to uninstall the VPN. So I deleted NordVPN, and the wifi was working fast on the new profile. Then went back to my main profile to start the process of migrating that account data to the new account, only to notice that the wifi was back to being fast on my old profile too now!

So tl;dr:

Try uninstalling your VPN. I have no idea what the problem was, but in the end, deleting my NordVPN app worked when none of the other solutions made any difference.

You are on the 5 GHz band, and you don't mention what is your Mac model. So maybe you have the same problem I had.

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; before, 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. Interestingly, an iPhone 7 and iPad Pro had no trouble with it.

(Some more context and info moved to a blog post)