I received a very good newsletter from wptuts+ with a tutorial about how to optimize a WordPress install and increase its performances.

Here are the headlines:

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Some of the factors affecting the performance of a WordPress site are:

Errors in the theme and plugins
Outdated code
Number of database queries
Lots of file requests
Slow server

Before we go further, let’s clear up two myths about plug-ins:

Plug-ins that are installed but not activated do not slow down or affect your website
How the plugins are coded affect your website and not how many plugins you have installed

Analyzing Performance of WordPress Website
1. Install the Tools We Would Need to Analyze Performance

2. Edit Your wp-config.php File

Open your wp-config.php and change the value of constant WP_DEBUG to true and below it add define(‘SAVEQUERIES’, true);

3. Test Your Theme for Errors

4. Check Website for Deprecated Calls

5. Use Debug Bar to Understand the Background Process

6. Analyze Impact of Plug-Ins

Using the data in the result you should now be able to understand and compare the impact of each plugin on the overall load times. Using advanced settings you can also change the test IP & log the last 100 visits.
Quick Tips on Improving Your Website’s Performance
1. Fix Errors & Avoid Use Any Deprecated Functions
2. Reduce MySQL Queries
3. Reduce Number of HTTP Requests
4. Use Asynchronous Script or Content Loading Wherever Possible
5. Use Only Absolutely Necessary Plug-Ins
6. Use a Caching Plugin

This is something what needs to be read, optimizing WP performance is a must.