Under a normal server load, WordPress is a fast and easy tool to publish blog posts and news articles. When the traffic gets thick, however, WordPress tends to get bogged down, and that can bring your website to a crawl.
To understand why this happens, it is first important to understand how WordPress and other content management systems (CMS) work. When you first publish a WordPress article, it does not create a static HTML file. Instead, it stores your article in a database. When a user requests the URL for that article, PHP accesses the database, retrieves the article, and then tells the web server to create a temporary HTML file for the user to view.
As you can imagine, a page that is accessed thousands or even millions of times a day would put a tremendous load on the server. That is where caching can help. With caching, pages that are accessed frequently are stored as static files on the server. Rather than having to go back to the database every time, even when a page has not been changed, your web server will simply retrieve the cached HTML file and display it for the user.
There are many cache plugins for WordPress, and I will leave it to you to decide the one that is best for your site, but caching is one possible solution to a slow blog with heavy web traffic. Many other CMS packages have cache plugins as well, and even if the one you use does not, you can use a free tool like xcache to cache dynamic PHP pages directly.