Joomla is one of the most user friendly Content Management Systems available today. However, it can suffer from the performance and page loadings.  Here are some points that should help to imporve performances.  Performance boost is also reported for Lighttpd (It's MUCH faster than Apache) server. But we find sometime it causes load spikes. So, you should use it after a few trial experiments. Most of the followings can be easily checked if you install firebug and yslow on your firefox browser.

Template Optimization

First and formost is the template optimization. There exist literally thousands of templates made specifically for Joomla, and most of them look quite nice, but most have some severe performance penalties.  The problem is that some of the templates out there use upwards of 70 separate images and many css style files to display the page.  Each image/css file generates a new request (which adds the network latency, usually between 0.1 seconds to as much as 1 second) and has to be downloaded.  The problem is exacerbated when those images are not as small (in terms of data size) as possible, and when they are not optimized.  Please check your page first using The Website Analyzer and reduce the number of objects from your template.

Be careful to choose 3rd party components and modules

Not all component and modules are optimized and are somtimes buggy. The problem is that some of the components are REALLY inefficient and wind up slowing your site down dramatically.  If you find page loading slower and your mysql queries increased then better drop the component/module.

Statistics will slow down the site

They are not worth it at all!  If you want nice stats, use Google Analytics to track your users.  Stay away from 3pd stats components.

Mambots/plugins can also make the site slow

Take a look at what mambots you are running.  If you are not using any of them, then get rid of them (don't just disable them, uninstall them).  Every time you load a page, mambots are called as much as 20 to 30 times.  If the code isn't 100% efficient and needed, it's a waste of power.  This doesn't mean that if you use the mambot, you should still keep it.  There are some mambots that are notorious for slowing a site down.  One of these is the AllVideos Plugin.  It's a great solution, but uses a LOT of queries to complete.

SEF is not Server Friendly

        One of the best things you can do to a site, is make the URL's look more better.  The Joomla core does an acceptable job, but there are a handful of 3pd components that make really nice URLs.  The problem with these components, is that they are not very performance friendly (they use a ton of queries).  I prefer OpenSEF to any of the others (But it's no longer actively developed) as it seems to be a good mix of power, flexibility and performance.  Keep in mind, that using SEF URLs is a tradeoff.  A good URL will look better to a search engine, and to end users, but it hurts the load time.  Think carefully before deciding to enable SEF URLs.  One thing you can do, is periodically optimize the database by purging unpublished and invalid URLs from the OpenSEF database (an option in the admin section of OpenSEF).  

Enable Caching as Much as Possible

        If you can, enable Joomla's core content cache.  It does speed things up when viewing content and modules (mambots don't need to be processed each time the page is loaded).  If you really want to speed things up for unregistered users, take a look at my  Page Caching component which caches entire pages.  This is probably the best performance gain you can find you unregistered users (the majority of users for 95% of websites).  This does not mean that you can forget about the rest of these optimizations, because the registered users will still see the uncached site.  

Find the Bad Queries

        From the Joomla config, enable debugging.  Then view your site, and check out the database queries being executed.  You want this number to be as low as possible (under 30 preferably, but under 50 or so should suffice).  Start by looking at the recurring queries (ones that are executed over and over).  You SHOULD be able to tell what extension is generating the queries by the table they look at (for example, "Select * from jos_opensef_config" is from OpenSEF).  There are two ways of dealing with queries that run often.  You can either disable that extension, or contact a developer to optimize the extension (either the developer of the extension, or a freelance developer like me).  

Stay Away from Bridges

        A popular method of incorporating functionality into Joomla is via a bridge.  This involves modifying the Joomla core and the core of another program (such as SMF, or Gallery2) to provide the functionality of both.  On the surface, this looks great because the other program is often much more refined and has more features than any "true" Joomla extensions available, but underneath is a different story.  The problem is that both are independent systems, which means each one needs to load its own "core" (including frameworks, user management, and more).  This has a HUGE performance penalty, because you are basically loading two pages for every page loaded.  The slight functionality loss of going from SMF to something like Fireboard is well worth the performance gains.

Optimize your Database

        The one thing that is put under the most strain in most Joomla installations is the Database server.  There are two main ways to optimize the database.  Optimize the tables, and optimize the schema.  The difference, is that the tables are how the tables actually store the data, and the schema is how they are supposed to store the data.  The schema only needs to be optimized once, while the tables need to be optimized regularly (as data is added, edited, and deleted).  To optimize the schema, please refer to the article  Improving Joomla's Queries.  To optimize the tables go into phpMyAdmin, select all tables, and click "optimize".  To read more about optimizing the database tables
 

Gzip might help

        Encoding your pages with Gzip is an 90% solution...  If your server's processor is faster than your bandwidth (again, 90% of the time), than enabling Gzip compression will actually speed up your site.  I say that because gzipping the pages does require some CPU horsepower.  Try enabling Gzip and see if your site loads any faster (especially on highly loaded servers).  If it doesn't, or you get CPU warnings from your host, disable Gzip.  It can hurt you, but most of the time it does help.  The only way to find out is to try it.