Technologies are described herein for collecting client-side performance metrics and latencies. A web page received by a web browser application executing on a user computing device includes markup or scripting code that instructs the browser to collect performance measures during the rendering of the content of the web page. The performance measures may include operation timings that measure the time it takes for a particular operation to complete during the rendering of the content and/or event counters that count the number of times that a specific event occurs during the rendering of the content. The web browser application sends an event report containing the collected performance measures to a reporting module executing on a server computer. The reporting module receives the event report, validates the content of the event report, and adds the event report to a database or other data storage system.