opencart site with over 150,000 items is very slow

We have over 150,000 products and 2,000 categories in our opencart store (version 1.5.2.1). The server is dedicated, 4 gigs of ram. The CPU is an Intel(R) Xeon(R) CPU E31270 @ 3.40GHz quad-core. We have caching on all pages. Product counts are disabled. The site is still very slow. Using the search takes about 1 minute to get results. When visiting a page with a lot of categories it can take up to a minute for pages to load. example: ([login to view URL]). When executing large scripts the site is unresponsive. Enabling page compression exhausts the server and crashes the site. How can we make the site faster even without caching? What can we do to speed up the site? We need someone with experience with large opecart sites to fix this for us. The site is [login to view URL] Here is another opencart site with over 100,000 items that is very fast. [login to view URL] How can we get our site to be as fast as that site? This job may require comunicating with our hosting company These are the two caching mods we have installed. [login to view URL] [login to view URL]