Joomla is quite efficent in terms of response time.

However, when your website becomes quite popular, the server load can slow down the overall experience. How can you improve the Joomla performance?

The Joomla Performance website lists some great ways to accomplish your needs, so it's worth to have a look at it.

I recently read an article on that website talking about the SQL index directive, not used in the standard Joomla installation. The SQL Index can dramatically improve the performance of load-stressed website. Here you can find the article about Improving Joomla's Queries.