When I make a search or just simply browse on my site it is really slow. I am using Zen Cart for my new Web Shopping.
My site is: http://shopfarmacia.it
A good place to start would be to check the page using some tools such as Google Pagespeed, Yahoo Y!Slow and Pingdom Tools and follow their recommendations for speeding up your site.
It doesn't seem like Zen Cart is much of a problem, every page you view requires you to download 700k of data, most of which isn't cached.