First, go to Admin Panel -> Settings -> Switches and see if any of the asterisked switches are on. These are known to cause slowness on large sites, so you may need to switch them off.

Next, check bot traffic. You may have bots that do you no good which are hammering your site at a fast pace and thus increasing server load. You could block such bots from part of your site with the options at Admin Panel -> Settings -> SEO to block bots from non-rewritten or non-critical pages, or you could block them entirely by user agent with the "other .htaccess content" field -- see www.thesitewizard.com/apach...k-bots-with-htaccess.shtml for a guide to the latter.

If you're desperate and getting occasional extreme loads, you can use the load management at Admin -> Settings -> System Configuration to lock out spiders or even guests when the load gets too high (note that this depends on having a Linux or Unix server, not available for Windows).

If particular pages are slow, look at whether some dynamic content on them can be made static.

If you're an advanced use, you can set debug mode to show queries to admin at Admin -> Settings -> System Configuration -> "Debug mode" and then view one of the slow pages to see which queries are slowing it down or if there are an extraordinary number of queries on the page.

Also feel free to contact WSN support for advice on your unique situation.