This trick solved many of the problems I had with Leopard: the 10 secs delay for Mail to check my email, slow connection on MSN Messenger, Adium won’t connect to MSN at all…
Two steps to fix this(these) issue(s):

1. go to Network > Advanced > DNS, and add these DNS: 4.2.2.2 & 141.1.1.1, click on “Apply” and close system preferences.
   
2. open Terminal (Applications > Utilities > Terminal.app), and type this command: sudo dscacheutil -flushcache

Now you can start your applications and you’ll feel the change!

Posted in Leopard, Macintosh Tips & Help