JavaScript is an increasingly pivotal part of the WordPress experience, especially in the admin interface. The upcoming REST API will open the door to many rich new client experiences and single-page app opportunities. You may be familiar with techniques for tuning server-side or network performance, but understanding client-side performance is just as critical and can no longer be ignored. Delight your users wherever they may be, by providing a speedy experience even on low-power mobile devices. This talk will cover key fundamentals of client-side performance, from JavaScript code execution to browser layout and the critical rendering path. Most importantly, we will cover the tools that you can use to discover, analyze, and fix browser performance issues.