In order to maximize performance of Drupal we need to understand all the spokes in the wheel that makes it run. Drupal is comprised of many layers that all come together, many of which are outside of the software itself. At the very basics there are two main types of traffic that Drupal can receive, each with very distinct performance attributes. It's important to understand the type of site you have and the type of users coming to your site.
- Anonymous Traffic
- Authenticated Traffic
Lighter user, basic configuration and performance tuning should handle more use cases.
Heavier user, can lead to more complex configurations to handle load and performance.
Please see the section below on Authenticated Users.
Drupal offers basic performance tuning at:
Administration > Configuration > Performance > Development > Performance (admin/config/development/performance)
It is recommended that block and page caching be enabled. Instead of Drupal dynamically performing complex SQL queries it caches these for quick retrieval.
If you enabled caching, remember, you may need to Flush Cache (Clear all caches button) to see any changes made to your site. The Devel module makes this option part of your top menu for quick cache flushing. If you're a more technical user look at Drush. You can flush cache with one shell command
drush cc all.
Typically during developing (on dev), caching should be disabled. On production sites, caching should always remain enabled for best performance.
Page and block caching is not always available. For example, page caching will not be available when a component of the page has marked the page as explicitly not cacheable, such as using sessions. Block caching will not be available when using a module that utilizes node access, such as Organic Groups or Taxonomy Access Control Lite.
Opcode Caching (PHP - high CPU)
PHP out of the box is a dynamic language and can lead to heavy CPU usage on web servers. There are multiple types of opcode caching add-ons for PHP available that will convert your .php page into memory (byte code) to provide a major benefit in load time and reduced CPU usage. Each of these will require root access to install and have their own specific configurations that will need some attending before using. Once configured and enabled they can provide a substantial benefit to a slow site and greatly reduce CPU usage from PHP.
Wikipedia has full list of other Opcode cache technologies for PHP.
Web Server (Proxy) Caching
If better optimization is required, you might install one or two performance modules. For example, if the site is on a shared server, try Boost (static page caching for non-logged in visitors), complemented with Authcache -in CR's file or db mode- or Advanced Cache (for logged in members). If on a VPS or dedicated server, there are several suitable options: Boost (for example on a low RAM VPS), Authcache, Cache Router (which includes , Memcache...), Varnish, etc. Memcache (or Memcache Storage) and Varnish are also especially suitable when load-balancing multiple servers.
For extensive and detailed information, and more options on server optimization for Drupal, see the following selection of resources.
Drupal performance resources
Information and tutorials on optimizing Drupal's performance, speed, and scalability.
General Drupal performance
- High performance - Drupal group dedicated to solutions and approaches for high traffic, high performing Drupal sites.
- Caching: Modules that make Drupal scale - Wiki page for comparison of performance and scalability modules.
- Server tuning considerations - Detailed collection of HowTo pages.
- Compatible with Memcache module? - Boost module issue including a solution to use several performance modules working together for logged in and non-logged in users.
- System: cron and caching - System, a required core module, provides system-wide defaults for storing (caching) web pages to improve efficiency, and performing other essential tasks.
- Throttle: congestion control - Throttle, an optional core module, provides a congestion control throttling mechanism for automatically detecting a surge in incoming traffic.
- Drupal guide to caching [draft 1]  - Preliminary versions, by Steve Dondley, for an introduction to the various caching mechanisms Drupal 6 uses to speed the site.
- Coding Standards and Performance Optimization - Group for Drupal developers, on optimized PHP coding.
- Performance and scalability - Deprecated forum with old threads; for current discussions see the two performance groups, specially High Performance for Drupal sites.
- Load Testing Drupal - A module that generates customized Apache JMeter and Selenium test scripts for any Drupal installation. These scripts are ready to run as part of a load test on a load testing cloud provided by BlazeMeter.
- ServerFault - which php opcode cache and why?
- Drupal performance - Articles by Drupal founder Dries Buytaert.
- A beginner's guide to caching data - An article by Jeff Eaton, of Lullabot.
- Drupal Performance and Scalability - Online book, freely available at Tag1; see also articles on their blog.
- Drupal performance tuning and optimization for large web sites - Extensive collection of articles at 2bits; for instance, "Tips on speeding up your Drupal sites".
- Links and resources on Drupal performance tuning and optimization - Some additional articles on the web; a selection by 2bits.
- Performance - Articles on Drupal performance by Wim Leers, including the detailed "Improving Drupal's page loading performance".
- Performance and Scalability - Module list at Drupal Modules.
- Scaling Drupal - An open-source infrastructure for high-traffic Drupal sites - Multiple server configurations.
- Pressflow - A derivative of Drupal core providing enhanced performance, scalability and data integrity for high traffic sites. Open source, from Four Kitchens.
- Cocomore Drupal Core- Another derivative of Drupal core with own improvements and other of the Drupal community (including Pressflow).
- LibreTechTips.com - "How caching saved my Drupal site and how I configured it".
- Phase2 - "Caching in Drupal".
Videos, screencasts and slideshows
- Mini-Lesson: Devel Module Quickie - A quick screencast showing off the virtues and power of devel.module, a true must-have for any Drupal developer.
- Caching and performance improvements - Video and article from Drupalcon Szeged 2008.
- Increasing Performance on your Drupal Site - Screencast at Drupal Dude.
- Drupal Scaling and Performance - A presentation and a video from the June 2008 LA Drupal Users Group meeting.
- Drupal Caching and Optimization Strategies - Slide show from Melbourne Drupal Mini-Conference, February 3rd, 2007.
- Staying online during a perfect storm of traffic - With links to "Improving Drupal's Performance with the Boost Module for the UN's Millennium Campaign" and other articles on that large international event.
- High volume Drupal sites - what do we need to know? - Thread including Intoxination's case study, with an average about a quarter million visitors a day and 400,000 page views.
- Digg Effect - Discussion on a case where normal cache (not aggressive) wasn't enough to deal with the Digg effect.
- Can a Drupal web site handle a million page views a day? - A case study by 2bits of a site that can handle this traffic on a single server, including a Digg front page; they use Memcache without the database caching.
- National Novel Writing Month and Drupal Scalability - Drupal scales really well, with a little help from performance modules, patches, etc.
- How to make Drupal run 8.5x faster in 5 minutes… - How a server using pretty large and hefty modules did it, by enabling Drupal’s built in caching, and installing APC.
- How I Survived a 2300% Traffic Increase with Drupal - A site on a shared hosting account, managing a traffic spike.
Monitoring and troubleshooting
- Drupal performance measurement & benchmarking - Methods to measure the performance of your web server.
- Devel - Module for developers that includes performance logging.
- ApacheBench - A module that provides both invocation of the ApacheBench program, as well as storing and reporting on cron-based invocations.
- Views 2.0 officially released! - Upon previewing, performance timers show how long your View took to build, query, and render; great for quick performance tuning.
- Benchmarking Drupal - A group.
- Drupal webserver configurations compared - Benchmarks measuring the performance of different configurations, by Dries Buytaert.
- Performance Logging and Monitoring: A module to log and monitor performance statistics for Drupal - This module is now part of the Devel module; article at 2bits, where more articles on monitoring, tuning and benchmarking are available.