Though this post might be dated as it concerns about Ubuntu Karmic Koala release (9.10), but nevertheless the problem can be considered as annoying and serious.
People who encountered this bug may report that web browsing speed is relatively slow compared to the internet speed as tested with http://speedtest.net, and some application which uses internet is behaving like it was having problems with network connection.
According to Ubuntu bug report #417757, this problem is caused by DNS resolver which attempts to request IPV6 AAAA record first. The problem is noticeable on application which supports IPV6, where the getaddrinfo() will ask for AAAA record which the DNS resolver does not understand. This will cause the DNS resolver to reject the request and caused a time out on the client side, which in turn cause the delay.
For a web browsing application like Mozilla Firefox, an average website would normally take about 4-10 DNS request and hence the effect would be more noticeable on web browser, making it look like the network is having problems with slow internet connections.
The solution for this problem is to install a local DNS resolver (sudo apt-get install pdns-resolver) and to edit the “/etc/resolve.conf” file, changing the ‘nameserver’ parameter to 127.0.0.1