Is your website slow?

Here is how to speed it up!

Google Webmaster Tools

Labs > Site performance

Read "Page speed suggestions"

See where your biggest bottlenecks are (identify that low-hanging fruit) before spending time on things you think are causing bottlenecks

xdebug

apt-get install php-xdebug

Simple & easy to setup

Webgrind

Separate install - just a directory with php code that you need to unpack  in a web root, makes viewing the generated xdebug data in a browser a pleasure

Saves the parse-and-compile step by doing it once and caches the result

XCache (the best)

Adds a read-through cache layer to MySQL

Memcached

Simple to setup

Enable the slow query log

Logs all distinct queries, how many times each is run and how long each takes to process

Figuring out how to identify memcache keys that need to be invalidated is non-trivial. Use the slow query log to see what is actually being run, then trace back to figure out which actions change data that other actions read

Install "WP Super Cache"

Completely eliminates PHP from cached requests by serving static HTML content

Use Ajax URLs and create an HTML snapshot

WP Super Cache does it for you

How to do it yourself