A performance budget is just what it sounds like: you set a “budget” on your page and do not allow the page to exceed that. This may be a specific load time, but it is usually an easier conversation to have when you break the budget down into the number of requests or size of the page.
(...)
Once those goals are set, you stick to them. Anytime you want to add something to a page, you need to ensure it stays within budget. Steve Souders talked about the three options you have if something does not fit within the budget:

1. Optimize an existing feature or asset on the page.
2. Remove an existing feature or asset from the page.
3. Don’t add the new feature or asset.

Le budget performance correspond à la valeur max autorisée pour une métrique de performance (temps de chargement, nombre de requêtes ou poids de la page). On peut ensuite tester le "budget" au cours de l'intégration continue avec un outils comme SiteSpeed.io et remonter une alerte en cas de dépassement.