A Google Page Speed developer is responsible for optimizing website performance. They utilize Google's PageSpeed Insights tool to analyze a website's load time and efficiency on both desktop and mobile platforms. The tool provides recommendations for improvement, such as reducing server response time, leveraging browser caching, optimizing images, minifying CSS, and eliminating render-blocking JavaScript. The developer implements these changes to maximize the website's speed and usability, leading to improved user experience, lower bounce rates, and potentially higher search engine rankings. They also monitor the site's performance regularly to ensure optimal speed and efficiency.