I've noticed that when I have Transmission running my network speed becomes very slow. Even if I'm only uploading data (the same is true even if the upload rate is low as 5KB/sec) via Transmission the download speed becomes very slow throughout the computer. It doesn't mattet if I'm browsing using Safari, streaming music in iTunes, if a software is upgrading itself etc they all become slow. As soon as I close Transmission everything speeds up again.

I'm connected through my Airport time capsule.


Edit: This is to clarify some things as mentioned in Tetsujin's response.

  • I only get this problem when Transmission is running. Even if Transmission is downloading 0 KB/sec and uploading at 3 KB/sec I still have these problems, so it's not because I'm reaching maximum bandwidth.

  • I'm actually running through a VPN so my ISP should not be able to see that it's torrent traffic (nothing illegal here, I'm using VPN because I like anonymity). Actually Transmission is running through a udp port that I opened via my VPN provider. I've also tried running it through a tcp port, same issues. I tried randomized ports, same issue.

  • I've tried using other torrent software and I think this actually might be due to Transmission specifically.

  • What is your "Normal" speed, what are your WiFi numbers/settings? if you try without VPN how does it look than? – Ruskes Mar 7 '15 at 17:20
  • @Buscar웃: I get the same issues w/o VPN. Say I'm downloading a big file via Safari: If I close Transmission midway the speed will increase about 2-3 fold. – turtlespeed Mar 7 '15 at 17:28
  • It's possible it's due to Transmission, but it's also possible that there are a large number of connections open (even though the bandwidth being used is small). This can slow down a router considerably, which would impact its ability to route ANY traffic. – tubedogg Mar 7 '15 at 18:44
  • I get the same issue with both Transmission and uTorrent on occasion. Full network speed returns as soon as whichever program I'm using is quit. I don't have to be actually downloading for this to happen, and as far as seeding goes I have the upload set to 50% of the available upload so there should be plenty left for page requests and other upstream data. I've always assumed it was due to the number of connections open, and generally worked around it by torrenting when I'm at work or asleep. – ScunnerDarkly Mar 7 '15 at 21:19

This is not a answer but a comparison.

I had to use it to show screen shoots.

First here is my basis: (my WiFi connection)


Second here is a sample when downloading two large files (at same time). As you can see the download did not budge, so I am not at the limit.

First file is 853 Meg size while second is 442 MB. Both are downloading with about 160 KB/sec.


And I am using Firefox to access the download sites.

However my example is strictly downloading. And there is no limit on download speed. How ever there is a speed limit to Upload. As you know Torrenting could be tricky since you are also uploading (sharing).

So if you can limit the number for Uploading, that also uses your Internet connection and can slow it down significantly.


Only when torrenting?

Did you check you aren't hitting your actual bandwidth limit, either up or down?
Either will hinder your speed.

If not, check you have QoS & WMM enabled on the wifi router. [idk how to do that on an Airport

Alternatively, your ISP may be actively limiting you when it detects P2P.

From http://www.wired.com/2013/05/optimize-video-stream/

WMM (Wi-Fi MultiMedia) is a ratified certification that prioritizes network traffic based on four types of traffic: voice, video, best effort, and background. In theory, WMM should route your traffic automatically based on traffic type. Apple’s Airport Extreme has WMM on by default, and you can’t turn it off. Linksys, on the other hand, gives you the ability to adjust WMM settings.

QoS (Quality of Service) does the same thing but at the application level. It also deals with the quality and predictability of networks, but you don’t really need to know any of that stuff. You just want to watch Downton Abbey without it looking like a copy of a copy of a VHS tape.

With QoS, depending on the router, you can prioritize applications or devices. The device prioritization will usually ask for MAC address. This is the unique number assigned to all devices for networking. You can usually find it near the Ethernet port, in a device’s settings, or the router will display it for all the devices connected to it. If your router allows it, prioritize both the application and the device to ensure you’re getting the best possible bandwidth within your home.

Further research tells me the Airport has neither of these…


The AirPort Extreme does not have parental controls, Wi-Fi Multimedia (WMM) or Quality of Service (QoS), which means that it is not able to prioritize different types of network traffic or guarantee predictable network behavior. It instead relies on the device to select the optimal band. It does have a USB 2.0 port for networked printers and network attached storage for Mac and PC users.

…so you might be stuck.


You don't mention your connection bandwidth, and that can be important. You should never use 100% of your upload bandwidth, or that will drag down your download speeds.

So, to check if this is the problem, can you answer this?

  • what up/download speeds are you supposed to get according to your provider?
  • what do you get in speedtest.net when no torrents are running?
  • what do you get in speedtest.net when torrents are running?
  • does the change happen as soon as torrents start running, or takes some time? how long?

Not really an answer (yet), but I can't post comments, so...

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