Improve Performance of a Drupal Site

Improve Performance of a Drupal Site (In the increasing order of complexity)

• Keep the core updated, contrib module and themes updated. Yes it matters.
• Install APC on your server. (Moved to top based on suggestion from Letharion)
• Page Caching : admin/config/development/performance Difference between Minimum cache lifetime and Expiration of cached pages
• Block Caching https://drupal.org/project/blockcache_alter Caching options for all the blocks.
• Aggregate javascript and css files - Front End Improvements https://www.drupal.org/project/advagg
• Disable Unnecessary modules. Every module adds to the amount of code that needs to be available for a page load. And it also increases the number of lookups. Wherver possible use a generic module in place of multiple module that does specific functionalities.
• Cache Views content - Content aware caching for Views https://www.drupal.org/project/views_content_cache
• Disable DB logging - Use https://drupal.org/project/syslog_ng
• Reduce 404 Errors - http://www.brokenlinkcheck.com/
• Fast 404 Responses - https://drupal.org/project/fast_404 - Try handling at server level.
• Client Side Validations - https://www.drupal.org/project/clientside_validation
• Compress Image - https://www.drupal.org/project/imageapi_optimize
• Lazy Loading of Images - Don’t load unnecessary images - https://www.drupal.org/project/lazyloader
• Use Sprite Sheets - https://www.drupal.org/project/spritesheets
• Set Minimum Cache Life Time Value to a higher number and use cache clearing modules to clear the caches for specific pages - Whenever I edit/update a node all the page caches for anonymous user are lost
• Use Devel Module to watch queries.
• Rewrite Views Queries / avoid Views if its a overkill.
• XHProf - https://www.drupal.org/project/XHProf
• FPM, HHVM.
• DB Profiling and Tuning - https://www.drupal.org/project/dbtuner
• Use Boost, don't Bootstrap DB if not required. https://drupal.org/project/boost For most of the small to medium sites Boost is good enough and you may not need Reverse Proxies or so.
• Use CDNs - https://www.drupal.org/project/cdn Its easy to set up.
• If your cache tables are huge use Memcached - If you can install memcached and set up RAM for it, it is not as complex as it sounds.
• Use Reverse Proxy Server - Varnish(at-least for assets). Helps a lot if most of your users are anonymous.
• Compressed transfer - Enable gzip compression
• Keep Alive - Use Persistent Connections where possible.
• Progressive JPEGS -
• CACHING IN CODE - Eaton’s blog is awesome. http://www.lullabot.com/blog/article/beginners-guide-caching-data-drupal-7
• Implement Cache Warming - https://www.drupal.org/project/cache_warmer - Cache Warm the pages before the end user hits them.
• Master Slave DB Config - https://www.drupal.org/project/autoslave makes it easier for you to set up one.
• Database Clusters - http://stackoverflow.com/questions/1163216/database-cluster-and-load-balancing
• Load Balancers -http://en.wikipedia.org/wiki/Load_balancing_(computing)
• Use heuristic Cache Warming - https://www.drupal.org/project/cache_graceful
• Authenticated User Caching - https://www.drupal.org/project/authcache