Brands that optimize speed will often find they regress quickly. This is because website performance is a lot like getting fit: it's not enough to make a one time effort; you have to change your lifestyle.
Performance budgets are one way to address this. A performance budget is a set of limits on metrics that affect site performance. The concept is similar to a financial budget: you set a limit and make sure you stay within it. In general, a good performance budget combines different types of metrics; so, for example, the performance budget for a product page might look as follows:
| Time To Interactive on slow 3G | Less than | 5 seconds |
| First Contentful Paint on slow 3G | Less than | 2 seconds |
| Lighthouse performance score | Greater than | 80 |
| Total JavaScript size | Less than | 170 kb |
Once set, a performance budget has to be enforced, which means for example incorporating the budget into your build process and reporting. Tools like Lighthouse can be included in your continuous integration, and you can write tests that fail a build if key metrics drop below a set threshold. Additionally, regular reporting through dashboards or summary reports can help with visibility and accountability. Pinterest are one example of a business that have implemented performance budgets to make sure their fast experience stays fast, while brands like Experian are now using site speed as a key metric in their monthly executive KPI reports.
You can find more details on performance budgets here. A guide that describes how to further instill performance culture and make speed metrics visible and tangible for all stakeholders can be found here.