I'm seeing slowdowns not just on my MBP running 10.6.4 and Safari 5, but also on any computer on my WiFi network.... only when I'm running Safari 5.
If I run Chrome or Firefox I dont see any network slowdowns. As soon as I start Safari 5, every computer on my network starts getting extremely long (several seconds) DNS lookups - if you try and go to any website you get several seconds pause and then suddenly it loads quick as it should do on my broadband.
I've run Wireshark on my network and when I start Safari 5 I start getting several 'Standard Query SOA local' DNS lookups a second. These don't always start as soon as I open Safari, but as soon as they start this is when my response times drop, presumably because my Mac is flooding my ISPs DNS servers with these bogus queries - it issues several a second continually until I restart Airport. Once they start closing Safari doesn't make them stop, but if I stop Airport and restart it, they won't start again until I start Safari again then they start again.
Safari 5 for me is unusable on my network. I've read elsewhere that some routers are not supporting Safari 5's DNS prefetching correctly and this is what is causing the slowdown issues. I've posted this on Apple's discussion site and on other forums and logged it as a bug from Safari so hopefully Apple will look at this issue soon.