Webpage load time greatly increase to around 35 seconds when files are uploaded on 2nd router

Question

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.

Answer 1

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.

Answer 2

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.