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I have two routers connected together via ethernet cable to provide better WiFi coverage over my house. I have two Mac desktop computers connected via ethernet to the primary router, and various wireless devices in the house such as phones and tablets typically access the Internet via the second router's WiFi.

Everything works fine, except that when these wireless devices are uploading data (such as when lots of photographs or videos are taken on a mobile phone, and Android sync uploads them to Google), then the Internet connection on my desktop computers becomes very slow: instead of webpages loading more-or-less instantly, all webpages now take 30 or 40 seconds to load. Even a simple webpage like the Google homepage can take over 30 seconds to load.

This slowdown only happens when data is being uploaded on the second router. It does not occur if data is downloaded. And it does not really occur if the wireless devices are uploading data through the WiFi of my primary router, only when they upload data through the second router. It also occurs in various browsers (Safari, Firefox, Chrome).

My primary router is a BT Home Hub 5, and the second router is a Netgear DGN3500. I have also used a Netgear WNDR3400 as the second router and had the same problem. This slowdown problem occurs whether I connect the routers together by LAN-to-LAN, or by LAN-to-WAN (on the WNDR3400).

In the dual router setup I have, the 2nd router has its DHCP server turned off, and both routers have their own unique LAN IP numbers on the same LAN network. The 2nd router has its gateway address set to the LAN IP of the primary router.

My Internet connection is via ADSL2+, but I have a pretty good download speed of around 20 Mbps.

Any suggestions as to what might be causing this page load slowdown, and what might be done to rectify it, are gratefully received.

EDIT: thanks to the help and suggestions of Appleoddity below, I realized this slowdown problem was actually occurring when uploading data on either router's WiFi, and this appears to be more of an issue connected to the asymmetric upload/download speeds of ADSL, rather than related to the fact that I have two routers connected.

  • See my answer here for an explanation of why uploading things kills internet connections. (Theres a section lower down that answer.) Also Google for "bufferbloat" which is related. – Mokubai Aug 29 '18 at 14:56
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Saturating your bandwidth should never cause huge latency spikes. Especially your case: saturating your upload bandwidth should never slow your download speeds.

If that ever happens, it’s a well known bug in routers and modems, called bufferbloat. The solution is to use a bufferbloat-savvy scheduling algorithm in your router. The most famous one is CoDel, now FQ-CoDel. It’s built into recent versions of most Linux-based open source router firmware distros, such as OpenWrt.

If you enable CoDel in your router, that solves bufferbloat for that router, but not for your other routers or modems. But there’s a trick to solve it for your whole home network. Put your CoDel router at the head of your network so all your traffic has to pass through it. Enable traffic shaping in your CoDel router and make it a slight bottleneck. That way it will have a chance to solve congestion problems before bloated buffer queues can build up on other devices on your network.

  • Very interesting, Spiff. So this may just be a bufferbloat problem with my particular router. That makes sense, as I found few other reports of this type of slowdown online, suggesting that most modern routers do not suffer with this bufferbloat issue. Unfortunately there is no OpenWrt for my router, the BT Home Hub 5 Type B, so it may mean getting a new router in order to fix the problem. Thanks very much for the bufferbloat info. There is a website on bufferbloat here, for others who might be interested: bufferbloat.net – Hip89 Aug 30 '18 at 4:06
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The problem here is that you have neglected the relatively low upload speed on your network. ADSL connections are not symmetrical. Most home internet connections are not symmetrical. That means that your download speed is far greater than your upload speed.

You also said,

This slowdown only happens when data is being uploaded on the second router. It does not occur if data is downloaded. And it does not really occur if the wireless devices are uploading data through the WiFi of my primary router, only when they upload data through the second router.

What does, “does not really occur” mean? What it means to me is that you experience the same problem on the primary router that you experience on the secondary router, albeit to a different degree.

What’s going on here is that when you upload large amounts of data you are saturating your internet connection. Uploading data has the ability to really cripple a slow internet connection. ADSL connections are known to have atrocious upload speeds.

You are not experiencing the issue on the primary router to the same degree as the secondary router for a number of possible different reasons. But the most likely cause is that the WiFi connection is not as good on the primary router which prevents the devices from saturating the connection as thoroughly as it does in the secondary router.

You can confirm what I have said in the following ways:

  1. Run a Speedtest on your internet connection using speedtest.net. Observe the upload speed is a mere fraction of the download speed.
  2. Run a speedtest while connected to each WiFi router. Observe that the speeds you get on one router are not the same as the other router.
  3. Run a ping test from any network connected device while another network connected device is uploading a large amount of data or during the upload phase of a speedtest. To do so, go to a command prompt / terminal and run the command ping 8.8.8.8 before and during a large upload. Observe ping response times increase drastically during an upload and you may even see “timed out” packets.

In this situation, you have three possible options:

  1. Replace or upgrade your internet connection for better speeds. I suggest replacing ADSL as it is known to have atrocious upload speeds and there is likely nothing to “upgrade” to that will improve that.
  2. Work with your ISP to confirm the connection is stable and clean and working properly and that you are getting the subscribed speeds.
  3. Implement bandwidth throttling and QoS at your routers or by using a 3rd party device. This will prevent any one device from saturating your internet connection. The methods of doing this are beyond the scope of this question / answer.
  • “Does not really occur” means that usually there is no page load slowdown at all when uploading data via the primary router's WiFi, though sometimes you do get a slowdown such that a page might take up to 5 seconds to load. – Hip89 Aug 29 '18 at 15:11
  • "Implement bandwidth throttling and QoS at your routers or by using a 3rd party device." The WNDR3400 router I have has QoS, and when I've throttled the upload speeds down to the minimum setting on that router (which is 64 kbps), unfortunately it makes little difference. – Hip89 Aug 29 '18 at 15:14
  • "But the most likely cause is that the WiFi connection is not as good on the primary router which prevents the devices from saturating the connection as thoroughly as it does in the secondary router." Both routers have the same WiFi speed. – Hip89 Aug 29 '18 at 15:15
  • @user3638245 run the tests I suggested and edit your original post to include the information about your results including the upload / download speeds under different scenarios and your ping results. – Appleoddity Aug 29 '18 at 15:16
  • Just performed a ping test to 8.8.8.8 on my desktop Mac, and can confirm that ping times increase from a norm of around 8 ms when no upload is taking place, to around 400 ms when the 2nd router WiFi is uploading data from my Android tablet. But here is a fascinating thing I just observed: when I moved the Android tablet close to the 2nd router, then ping times dropped back down to around 8 ms; and when I moved the tablet far away from the 2nd router, at the other side of the house, then ping times increased to over 2000 ms. So this is very strange. – Hip89 Aug 29 '18 at 15:32

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