For large wordpress sites or sites that are expierencing slow load times or high server load one of the cheapeast and easiest ways to improve performance is by installing one of the many Wordpress caching plugins. Some are more advanced than others, but the basic idea is generally the same. If you have mostly static pages then you want to try to reduce MySQL queries to pull each post every time someone visits your site. Caching basically allows for creation of static HTML pages on your hosting platform thus bypassing expensive database queries and PHP scripts. The trade off is that is uses slightly more disk spaces to store those pages, but the benefits are immense in terms of performance and how your users view how fast your pages load. If you have a highly dynamic site in which your posts are constantly changing you might not see much benefit, but that is rarely the case with Wordpress type Blog sites.
There are many caching plugins available via Wordpress, but here are 2 that we can recommend, try both and see which meets your needs better.