This article is part of a series on WordPress Optimization.
The first and easiest way to improve WordPress performance is plugins. Deactivate and delete any unnecessary plugins. Try selectively disabling plugins to measure server performance. Is one of your plugins significantly affecting your site's performance?
Then you can look at optimizing plugins. Are plugins coded inefficiently? Do they repeat unnecessary database queries? WordPress has its own caching system, so generally speaking, using functions like get_option(), update_option() and so on will be faster than writing SQL.
After plugins come theme optimization.
We have seen themes which put 3x extra load to the server. Turned out it caused 3x more database queries - which is bad in itself. Then we found out that some of the queries are unoptimized. Not good.
You can also use offloading to optimize your theme.
WordPress is fast, but it contains a lot of code that needs to be loaded each and every time a page is shown. (And some plugins like bbPress contain a lot of additional code.)
In some environments this load time can dominate performance times, adding several seconds per page. Op Code caching could help, but will not run in SuPhp shared hosting environments.