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I have a Mac running Mojave 10.14.5.

I have an issue with the home Wi-Fi. I have a 400Mbps internet connection. When I start the laptop, the Wi-Fi is fine and I'm getting expected internet speeds (as per fast.com), but at some point it drops to less than 100Mbps (again, as per fast.com), and somehow this affects all other devices (phone, tv) connected to the same Wi-Fi.

But then just after I restart the Mac, the speed goes to 400Mbps again, on both the Mac and the other devices.

How can I troubleshoot this situation?

Edit: I added and Image of how things look when connection drops.

enter image description here

  • how are you determining the speeds? – jmh Jul 11 at 23:19
  • I am doing a speed test on fast.com – callback Jul 11 at 23:30
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    Seems like you have a soft that hogs all of the bandwidth available on your network… Try and open Activity Monitor whenever the your connection is slow and see if there is a process uploading or downloading a lot. For more troubleshooting options, I recommend the excellent (albeit not free) Little Snitch. – Frizlab Jul 14 at 12:11
  • @Frizlab , I installed the app, and checked when the connection drops, but it showed that nothing suspicious is using any bandwidth, apart from the few megabytes used by Chrome when I am browsing. I changed the DNS of the wifi in the network configuration to 8.8.8.8, and instead of dropping until 100, now it drops to ~250 Mbps. but of course as soon as I restart the laptop, the speed goes back to over 400 Mbps. Any further idea please? – callback Jul 16 at 19:07
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    @Frizlab the culprit was a VPN software. Even though It wasnt open, but just the fact that it was present on the system caused the issue. Uninstalled and everything is back to speed! Thanks for the support! – callback Aug 4 at 6:48
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WiFi devices will reduce speeds if they detect too many errors at the higher rates. For example, in your screenshot, your WiFi radio has negotiated a fairly decent RF speed (MCS 9, 867Mbps) with the router. (It would be interesting to see what the router says, as Tx and Rx can differ.)

Over time, radio link quality changes, causing the radios at both ends to adjust Tx/Rx rates. If too many packets error out, the radios will back off to lower rates. If the error levels drop, the radios will (eventually) negotiate back up to better speeds. Rebooting it just resets the error counters; it's not actually fixing anything.

When your speed seems slower, check to see what rate the radio says it's negotiated with the router (and check the router's view of your laptop too). If it's lower, there's a reason, such as interference or another RF problem.

The 802.11ac standard also supports MIMO (multiple in, multiple out), which requires all devices to utilize more than one antenna to get maximum throughput (which is somewhere above 1Gbps I believe). There could be a situation where the AP radio is negotiating just a single channel with your laptop and multiple channels with other devices that come on after it does.

Without knowing what kind of router you have or how many other devices are on the network, it's hard to say what else could be causing the slow-down's/back-off's.

On most routers there are advanced wireless settings that can affect how clients interact with the router. In my experience, it is useful to disable the fancy settings (or setting them to defaults), then test each one until you find the culprit.

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I've had this problem on PCs too. Turned out to be my router. I replaced it with a Ubiquiti Unifi LR Access Point and that fixed all of my issues. The Unifi is a little harder to setup because it has a software controller but the speeds are insane. I get 50-75 Mbps downloads frequently.

The TX rate in the picture you posted is literally 108.37 MB/Sec so it looks like it is communicating fine but like I said mine looked good too... but it wasn't.

If you think it might be the AP try using WireShark to watch for bad packets on your wifi interface. Like Bryan said, that could be the cause of your slow down. If you're getting bad packets replace the router.

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I found the culprit. I had a software called Pulse Secure (to connect to my workplace). I uninstalled it and BOOM! Problem fixed. If you have the same problem, it is most likely some VPN software (even if not connected to it)

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