Wordpress relies heavily on MySQL and this can lead to a lot of load on server end when there is heavy traffic on the website.  To lessen the load, cache is recommended.  There are tools that can be used to cache the content.  We highly recommend that you install the following tools if you are using Wordpress -

http://wordpress.org/plugins/w3-total-cache/
http://wordpress.org/plugins/wp-smushit/