To improve your Joomla website's performance, just turn on "caching" on the "Cache" tab from Joomla's Global Configuration. When caching is turned on, Joomla creates a static file or "cache" of your website data on the disk. When the static file is found, the system retrieves the data from the disk rather than from the database. This improves performance because it saves system resources by not making redundant SQL calls to the database. And it will also speed up the display time of the pages.
You can set cache refresh rate in seconds. If you don't constantly update your contents, you might want to set it longer. And to further improve it, go to all your modules configurationand select "yes" to cache option (not available in all modules).