When you start to improve the performance in any application, is very difficult to validate this values, because new features is adding constantly and new packages from npm. If you don't have a validation to check the current performance very often, the value is decrease similar when you improve the accessibility, one month later the accessibility decrease
Tutorial: https://medium.com/@mtorre4580/performance-budget-for-next-js-e34eb4fda11e
The first step is create a performance budget, is a way to prevent the application decrease the performance, It can apply a js, images, css, metrics of core web vitals
The example is for Next.js applications but is similar for any framework
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bundlesize Library to allows check the current size of your bundle and define a threshold to limit if exceed
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@next/bundle-analyzer Wrapper of webpack-bundle-analyzer for Next.js to analyze the current bundle
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@lhci/cli CI for lighthouse to apply a performance budget with your current metrics to check