If it's your own server and you know that is has plenty of resources then something is definitely wrong. DNN and the DNN menus aren't the lightest, but they aren't as slow as all that either.

I dont know what to suggest though.. go through the server, clear out logs, clear out temp folders, check the size of your db, dump the transaction log, event logs, schedule history. How well does it run from localhost.. load it up in IE on the server. Check out Yslow in Firefox and see that that tells you. Watch the task manager while you use the site, see if the app pools are recycling a lot for any reason.... keep digging.

Maybe you can put your sites links for other's reference....

I felt that core team seems didn't met this serious issue yet...

I am trying to hidden some menu items and using links to avoid this issue now.

ps: I want to share some information I got from Taiwan....In Taiwan, the government's website project is not allowing using javascript menu in the site(I don't know why, but this is a requirement in the contract)...All government's sites built by DNN remove their DNN menu and using links to replace it...Sounds a little funny....

No it is definitely not a server issue. I'm experiancing the same problems on my local server running just that one DNN application over a 100Mbs ethernet connection. Most intersting is that the menu is only slow on the home page. All the other pages work fine.