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I am using a 15" and a 13" MacBook Pro with these profiles:

enter image description here

They're both connected to the same wifi network and they're at the same distance from the router.

Problem is that the 15" has a slower network. Sometimes youtube videos buffer. And generally feels less snappy. And as can be seen from the image above - and the speed tests on the right -, the network has these "moments" where it drops all of a sudden on the 15" (you can see the drops in the top orange graph), whereas it remains very stable on the 13".

The only major difference I was able to find between the two is that one uses DFS and the other doesn't (as pointed by the arrow).

Would anybody know why I'm getting lower speeds on the 15" and is there a way that I can also make it connect using DFS?

Note that my iPhone also seems to work with stable speed tests and no "drops".

EDIT What I've noticed is really a problem with these so called "drops". The network sort of stalls in the speed test run on the 15". Here's a few randomized speed tests on the 15":

enter image description here enter image description here

What I want to highlight here is not the speed but the "flat lines". When I see the speed-test run in real time I can really notice the number stalling - as it's waiting for something, and then all of a sudden dropping.

On other devices I really don't notice these kind of stalls; sure there are sometimes slower speeds but the graph is drawn fluidly and I don't notice so called "flat lines".

Here's a few test runs from another well-functioning device (an iPhone) showing how "the stalling" never seems to occur (there are no flat lines followed by sudden drops):

enter image description here enter image description here

As a last resort I also uninstalled a VPN but that didn't make a difference (the graph above was drawn after uninstalling the VPN software).

EDIT 2 Note that also if I plug the 15" to the router directly using ethernet, the "stalling" doesn't happen anymore and I get fluid speed tests.

EDIT 3 The slow 15" is a work laptop. I just realized there might be something installed that could cause this network stalling. I have uninstalled all software relating VPN or antivirus so I'm not sure what it could be. Thing is when I plug in ethernet I don't see these stalls so my feeling has more to do with something relating the wifi configuration.

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  • The DFS won't be of any use (it does permit your WI-Fi card to use channels which might be reserved for radars). Here, your 2 MBP are on the same channel: 120, and from the radio stand point, the MBP 15" has the best radio channel (585 Mbps > 468 Mbps). Then the difference doesn't come from the Wi-Fi but from another difference between these 2 Macs. Most probably from something using heavily the network or the I/O capacity of the MBP 15". If you want to make a valid performance comparison, you will have to be careful to run the same things on your 2 MBP. Check: peripherals, sharing, local I/O.
    – dan
    Commented May 14, 2020 at 13:06
  • @dan makes sense. Any clues how I can go about finding the root cause? Commented May 14, 2020 at 17:04
  • @dan to add, if I connect the 15" to an ethernet it works smoothly. So I'm thinking it's not the network being "used" by another program, or could it be anyway? Commented May 14, 2020 at 17:34
  • Have you checked the DNS server settings on the Wi-Fi network settings of the 15"? I seem to remember reading recently that if they're set to the same as your router's DNS servers it can cause slowdowns. Commented May 15, 2020 at 17:40
  • I've tried also changing DNS server to 8.8.8.8. Not much changes Commented May 16, 2020 at 20:39

2 Answers 2

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As with all mysteries such as these, it is best to narrow the tree of possibilities with a sort of binary search, as it were.

Here are some starting point (and if you're lucky, ending point) tests to eliminate whole branches from the tree:

  • Test a wired connection. You've already done this test ✅, so now we know:

    • It's not a hardware problem with the possible exception of wifi-specific hardware, which is highly unlikely, especially given the symptoms you report.
    • It's not an OS problem, again with the possible exception of wifi-specific components, which I think is also unlikely.
  • Boot your Mac in safe mode. Run the speed tests again. If the problem is gone, follow the steps under If the issue doesn't continue in safe mode at the provided link. If it is NOT gone, I would hold off on OS reinstall recommended by If the issue continues in safe mode fo now, trying the things below first.

  • Test it on an entirely different WiFi network. This might narrow it down to your router or how your Mac connects to that router via WiFi. If it turns out to be this, report back and we can go from here.

  • Test whether the flatlining is system wide when it occurs. First open a browser window open with a quickly loading web page already loaded. Run a speed test. When it flatlines and while it is flatlined, refresh the webpage. Does the webpage refresh stall too? It would be best if your speed test is not running in the same process as your web page test. If it is browser based, use a different speed test that isn't, or use different browsers (e.g. Safari and Firefox). Alternatively, use something other than a web page to see if the speedtest stall is a system-wide stall.

  • Test using a different browser, or a non-browser-based speed test.

I can come up with more but let's start with these.

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I experienced the same problem (high ping - hundreds of ms) and failing network connectivity on my macbook when my router switched to channel 100 on 5GHz. By disabling "Auto select channel including DFS channels" I solved the issue.

There are 16 DFS channels: 52, 56, 60, 64, 100, 104, 108, 112, 116, 120, 124, 128, 132, 136, 140, and 144. (DFS channels)

When I noticed the network is slow again I checked which channel is being used at the moment and it was channel 100 in ma case. I tried to manually select "non-DFS" channel and the ping was totally fine again!

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