Transmission makes my connection really slow

When Transmission is running, all the other apps seem to have a very low bandwith, even when Transmission uses only 10 kb/s.
It looks like the DNS resolution or something at the beginning of the connection is taking forever (Firefox takes 1 or 2 minutes to reach a domain before even starting to download the page.)

Here is a ping without Transmission running :
pierre@pierre-MacBook:~$ ping launchpad.net
PING launchpad.net (91.189.89.223) 56(84) bytes of data.
64 bytes from launchpad-net.banana.canonical.com (91.189.89.223): icmp_req=1 ttl=53 time=40.5 ms
64 bytes from launchpad-net.banana.canonical.com (91.189.89.223): icmp_req=2 ttl=53 time=33.4 ms
64 bytes from launchpad-net.banana.canonical.com (91.189.89.223): icmp_req=3 ttl=53 time=32.5 ms
64 bytes from launchpad-net.banana.canonical.com (91.189.89.223): icmp_req=4 ttl=53 time=42.0 ms
64 bytes from launchpad-net.banana.canonical.com (91.189.89.223): icmp_req=5 ttl=53 time=39.3 ms
64 bytes from launchpad-net.banana.canonical.com (91.189.89.223): icmp_req=6 ttl=53 time=40.8 ms
64 bytes from launchpad-net.banana.canonical.com (91.189.89.223): icmp_req=7 ttl=53 time=34.8 ms
64 bytes from launchpad-net.banana.canonical.com (91.189.89.223): icmp_req=8 ttl=53 time=50.5 ms
^C
--- launchpad.net ping statistics ---
8 packets transmitted, 8 received, 0% packet loss, time 7010ms
rtt min/avg/max/mdev = 32.506/39.266/50.523/5.446 ms

And here is a ping with Transmission running :
pierre@pierre-MacBook:~$ ping launchpad.net
PING launchpad.net (91.189.89.223) 56(84) bytes of data.
64 bytes from launchpad-net.banana.canonical.com (91.189.89.223): icmp_req=1 ttl=53 time=36.8 ms
64 bytes from launchpad-net.banana.canonical.com (91.189.89.223): icmp_req=2 ttl=53 time=35.1 ms
64 bytes from launchpad-net.banana.canonical.com (91.189.89.223): icmp_req=3 ttl=53 time=36.3 ms
64 bytes from launchpad-net.banana.canonical.com (91.189.89.223): icmp_req=4 ttl=53 time=35.8 ms
64 bytes from 91.189.89.223: icmp_req=5 ttl=53 time=36.8 ms
64 bytes from launchpad-net.banana.canonical.com (91.189.89.223): icmp_req=6 ttl=53 time=32.7 ms
64 bytes from launchpad-net.banana.canonical.com (91.189.89.223): icmp_req=7 ttl=53 time=47.7 ms
^C64 bytes from 91.189.89.223: icmp_req=8 ttl=53 time=34.3 ms

--- launchpad.net ping statistics ---
8 packets transmitted, 8 received, 0% packet loss, time 45519ms
rtt min/avg/max/mdev = 32.778/36.987/47.734/4.259 ms

(Look at the total time !)

It seems to affect the computer only and not the network because when Transmission-daemon is running on my second computer, I can browse just fine.

I have tried to reduce the max speed and the number of peers but it still does that.

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: transmission-gtk 2.33-0ubuntu2
ProcVersionSignature: Ubuntu 3.0.0-16.28-generic 3.0.17
Uname: Linux 3.0.0-16-generic i686
ApportVersion: 1.23-0ubuntu4
Architecture: i386
Date: Thu Feb 23 00:13:08 2012
SourcePackage: transmission
UpgradeStatus: No upgrade log present (probably fresh install)