Joomla is very slow

Hello guys

I'm using joomla 1.5 with my own template. In there are more than 13.000 articles.

Recently i decide to upgrade for joomla 1.7 and selected Sporter template.

I have used jupgrade to migrate the articles, categories and all things...

But when i put online my website and people begin enter they becomes very very slow. I don't know how can i optimaze then more.

I have +/- 300 people online simultaneously.

You can see the old site here:
http://www.setelagoas.com.br

and new site here:
http://saturn.setelagoas.com.br

I'm at 3 weeks trying to optimaze then.
If you guys can help me.

Sorry for the bad English.

You can use CDN/Cloud to speed it up, and also compression, takes some work but it's the best option. I did this on 3 sites already.

Do you make a css gziped?
I tried to do this, but in some browsers the javascript don't work when they are gziped.

How can i compress the files....

And what is CDN/Cloud??