After installing Kubuntu 9.10 RC1 networking became kind of flaky. It somehow worked but everything was slow and unpredictable. It was most visible while browsing the web. Some pages where loading for ages (5+ minutes), some never finishing to load at all while others were just fine after some initial delay. This problem was not only visible in firefox but also in konqueror.
I started wireshark to have a look on what was actually going on. It showed that my computer was sending DNS queries to my local DSL router (SpeedPort W303V; an ISP provided blackbox). This DSL router also works as a local DNS-cache which it is advertising via DHCP. This explains why the DNS queries are sent to the DSL router. The traffic log showed further that requests for A records (IPv4) were properly answered but requests asking for AAAA records (IPv6) were never blessed with a reply. The missing answer was triggering some retry mechanism which was resending the AAAA record request a few times. Only after this mechanism timed out, the browser was actually starting to load a page.
This nicely explains why some pages were loading fine after some initial delay (all content on one server) while others were loading forever (complex sites were a lot of addresses need to be resolved to load all content). It also explains why firefox and konqueror were both affected, DNS resolving is typically handled by the libc.
The DNS cache in my DSL router is apparently not handling requests for IPv6 address records. So far i was not able to figure out what exactly has changed between Kubuntu 9.04 and 9.10. Just replacing the /etc/resolv.conf with a version pointing to an external DNS server solved the problem for now.
Apart from that Kubuntu 9.10 looks very promising. A lot of the KDE4 problems which plagued me in 9.04 are gone.