Use The Index, Luke! : A tutorial covering SQL indexing and SQL tuning for developers: avoiding unnecessary details about database internals.
SQL indexing is the most effect...
MySQLTuner : MySQLTuner is a script written in Perl that will assist you with your MySQL configuration and make recommendations for increased performance and stabi...
Blackfire : Blackfire Profiler automatically instruments your code to gather data about consumed server resources like memory, CPU time, and I/O. But Blackfire Pr...
Vitess is a database clustering system for horizontal scaling of MySQL.
Let's make the web faster : There are many ways to make websites run faster. In this section, you can discover performance best practices that real web professionals employ in th...
Minimize round-trip times : An article on the best practices to apply in order to minimize HTTP round-trip times
WebPagetest : Run a free website speed test from multiple locations around the globe using real browsers (IE and Chrome) and at real consumer connection speeds.
Squoosh : Squoosh is an image compression web app that allows you to dive into the advanced options provided by various image compressors.
boomerang : boomerang is a piece of javascript that you add to your web pages, where it measures the performance of your website from your end user's point of vie...
Nice write-up on techniques for scaling up a web application.