A performance budget is a set of limits imposed on metrics that affect site performance. This could be the total size of a page, the time it takes to load on a mobile network, or even the number of HTTP requests that are sent. Defining a budget helps get the web performance conversation started. It serves as a point of reference for making decisions about design, technology, and adding features.
Having a budget enables designers to think about the effects of high-resolution images and the number of web fonts they pick. It also helps developers compare different approaches to a problem and evaluate frameworks and libraries based on their size and parsing cost.