It is not the standard way adsl works but it is possible you are on some other method of connection..
Please post the traceroute for us to see.
So my traceroute to 8.8.8.8 look like this.
Miniray:~ Ray$ traceroute 8.8.8.8
traceroute to 8.8.8.8 (8.8.8.8), 64 hops max, 52 byte packets
1 192.168.2.254 (192.168.2.254) 3.633 ms 0.714 ms 0.667 ms
2 adsl1.mel02.eftel.com (203.123.69.176) 6.826 ms 7.443 ms 6.757 ms
3 te0-1-0-308.core0.mel02.eftel.com (203.123.72.73) 7.183 ms 6.831 ms 7.221 ms
4 ve420.er01.mel01.eftel.com (14.137.114.25) 7.547 ms 8.387 ms 7.115 ms
5 eth2-1.bdr01.syd02.eftel.com (202.62.143.121) 22.017 ms 18.622 ms 18.649 ms
6 as15169.ve223-vpls138.bdr01.syd02.eftel.com (202.62.154.118) 19.923 ms 22.621 ms 20.027 ms
7 72.14.237.21 (72.14.237.21) 18.747 ms 18.394 ms 18.940 ms
8 google-public-dns-a.google.com (8.8.8.8) 19.115 ms 22.314 ms 19.126 ms
And let me go back to the start..
If you always have it use the secondary DNS.. put the google address as the primary and the address you want to use as secondary.. Does that trick the Airport to use the secondary DNS.. ??
If not then please also post a traceroute to whoever you are using for DNS other than google.
If it is slow.. the Airport may simply swap to google because it is faster.. the issue has nothing to do with position it is because the primary dns is not resolving the address quickly enough.
Therefore leave out the secondary DNS and just force it to use the primary.