Does iOS slow download speed when downloading torrents?
I don't know of any traffic shapping or bandwidth throttling capabilities of iOS.  Technically, you would need stateful packet inspection to be able to detect Torrents and then slow them down (traffic shaping); this is found on many advanced routers and most definately not in iOS.

Most likely, it's your ISP that's doing the throttling.


  Hundreds of larger and smaller ISPs all over the world try to limit
  BitTorrent traffic on their networks. Unfortunately, most companies
  are not very open about their network management solutions, with
  Comcast as the prime example.


Read the full article here.


What you want to do is Test to see if your carrier is blocking/slowing down your traffic.

Test if your ISP is blocking Bittorrent

This is a Java applet and does not require a Bittorrent client.  It will compare the download speed of a small test file seeded through Bittorrent with the download speed of the same file through normal file transfer.  If the difference is significant it will indicate that your ISP is throttling your file transfer.