WordPress by default, is not optimized for speed. It only uses a caching system to aid in creating copies of web pages and saving them as static files in databases. So, whenever a user re-visits your blog, a cached copy is opened; thereby, increasing page load speeds. If you want your pages to load faster so that more and more visitors will be interested in your blog or website, these WordPress plugins can help with that.
If this is not enough, here is a list of the top 5 plugins to help increase the speed of your website.
1. Cache Total W3
2. Cache Hyper Extended
3. Quick Cache
5. Cache DB Reloaded Fix
If you want to know the difference of these plugin and what it can do to your site, read the article here: http://skyje.com/plugins-speed-wordpress-website/