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Performance Budgets (budget.json)

Use a performance budget to assert thresholds for performance metrics. budget.json supports three types of budgets:

  • Timing budgets: Assert thresholds for time-based performance metrics like First Contentful Paint, Maximum First Input Delay, and Speed Index.
  • Resource counts: Assert thresholds for the quantity of resources on a page. These thresholds can be defined per resource type or for the page overall.
  • Resource sizes: Assert thresholds for the transfer size of resources on a page. These thresholds can be defined per resource type or for the page overall.

If performance budgets have been configured, the Lighthouse report will include a "Performance Budgets" section.

To make performance budgets a part of your CI process, refer to Lighthouse CI.

Usage

Performance budgets are supported in the CLI version of lighthouse. To use:

  1. Create a budget.json file.
  2. When running Lighthouse from the command line, pass the --budget-path flag followed by the path to budget file in order to calculate whenever a category is over budget.
lighthouse https://youtube.com --budget-path=budget.json

budget.json

The budget.json file is an array containing one or more Budget objects.

[
  {
    "path": "/*",
    "options": {
       "firstPartyHostnames": ["*.my-site.com", "my-site.cdn.com"]
    },
    "timings": [
      {
        "metric": "interactive",
        "budget": 5000
      },
      {
        "metric": "first-meaningful-paint",
        "budget": 2000
      }
    ],
    "resourceSizes": [
      {
        "resourceType": "total",
        "budget": 500
      },
      {
        "resourceType": "script",
        "budget": 150
      }
    ],
    "resourceCounts": [
      {
        "resourceType": "third-party",
        "budget": 100
      }
    ]
  },
  {
    "options": {
       "firstPartyHostnames": ["*.my-site.com", "my-site.cdn.com"]
    },
    "path": "/checkout",
    "resourceSizes": [
      {
        "resourceType": "script",
        "budget": 200
      }
    ]
  }
]

Further Explanation

Timing Budgets

Lighthouse 6 & up

Use the optional timings property to define budgets for time-based performance metrics. In this context, budgets are defined in milliseconds.

"timings": [
   {
         "metric": "interactive",
         "budget": 5000
   }
]

Supported timing metrics:

  • first-contentful-paint
  • first-cpu-idle
  • interactive
  • first-meaningful-paint
  • max-potential-fid
  • estimated-input-latency
  • total-blocking-time
  • speed-index
  • largest-contentful-paint
  • cumulative-layout-shift

Resource Budgets

Use the optional resourceSizes property to define budgets for the size of page resources. In this context, budgets are defined in kilobytes.

"resourceSizes": [
   {
      "resourceType": "script",
      "budget": 300
   }
]

Use the optional resourceCounts property to define budgets for the quantity of page resources. In this context, budgets are defined in # of requests.

"resourceCounts": [
   {
      "resourceType": "script",
      "budget": 10
   }
]

Budgets can be set for the follow resource types.

  • document
  • font
  • image
  • media
  • other
  • script
  • stylesheet
  • third-party
  • total

Using the path property

Lighthouse 5.3 & up

The path property indciates the pages that a budget applies to. This string should follow the robots.txt format.

If path is not supplied, a budget will apply to all pages.

If a page's URL path matches the path property of more than one budget in budget.json, then the last matching budget will be applied. As a result, global budgets (e.g. "path": "/*") should be listed first in budget.json, followed by the budgets that override the global budget (e.g. "path": "/blog").

Examples:

"path": "/"

Result: Matches all URL paths. This is equivalent to writing "path": "/*"

"path": "/articles"

Result: Matches all URL paths starting with /articles. For example, /articles and /articles/jun_02_2019.html

"path": "/store/*/details$"

Result: Matches /store/clothes/item123/details but does not match /store/details.

Identification of third-party resources

Lighthouse 6 & up

options.firstPartyHostnames can be used to indicate which resources should be considered first-party. Wildcards can optionally be used to match a hostname and all of its subdomains.

If this property is not set, the root domain and all its subdomains are considered first party.

"options": {
   "firstPartyHostnames": ["*.my-site.com", "my-site.cdn.com"]
}

Examples:

"firstPartyHostnames": ["pets.com"]

Result: pets.com is considerated first-party, but fishes.pets.com is not.

"firstPartyHostnames": ["*.pets.com"]

Result: Both pets.com and fishes.pets.com are considered first party.