Large Resource Auditor

Performance Auditor — flags page resources whose size is at or above a warning limit (500 KB by default) or a fail limit (1000 KB by default), both set per monitored URL.

Performance

Oversized assets delay rendering and inflate data transfer, especially on mobile.

Configurable

Both size limits are per-URL preferences, so a media-heavy site can raise them without changing the auditor.

This auditor loads the monitored page in a headless browser, collects every resource referenced by img, script, link[rel=stylesheet], video and iframe elements, and reports each one whose size is at or above the warning limit: as a warning while it stays below the fail limit, and as a failure once it reaches the fail limit.

How it works

Threshold check — before loading anything, the auditor checks that sizeLimitBytesWarning is lower than sizeLimitBytesFail. When it is not, the audit fails with a single check that names both values, and nothing is measured.
Page loading — opens the URL with Playwright (Basic auth is honoured) and waits for the load event. A page that answers HTTP 403, HTTP 429 or a Cloudflare challenge is reported as blocked; any other HTTP error, or a browser error, ends the audit with an ERROR.
Resource discovery — reads the src / href of img, script, link[rel=stylesheet], video and iframe elements. Relative and protocol-relative URLs are resolved against the final page URL; data:, blob: and other non-HTTP URLs are skipped.
Size lookup — sends an HTTP HEAD request per resource and reads Content-Length. When the answer has no Content-Length (chunked or streamed responses) or the HEAD fails, a GET downloads the body, up to sizeLimitBytesFail, and counts its bytes; a download that reaches the limit is reported as “at least” that size. Resources are measured eight at a time, each request limited to 5 seconds. A resource that neither request can size is reported as INFORMATION.
Comparison — every resource whose size is at or above sizeLimitBytesFail produces one FAIL check; every resource at or above sizeLimitBytesWarning and below sizeLimitBytesFail produces one WARNING check. Each check carries the resource URL, its size and both limits in data (sizeInBytes, sizeIsLowerBound, sizeLimitBytesWarning, sizeLimitBytesFail).

Preferences

Key	Type	Default	Description
`sizeLimitBytesFail`	integer (bytes)	`1024000` (1000 KB)	Resources at or above this size fail the audit. Set it per monitored URL on the Configuration tab; plain integer bytes only (for example `1048576` for 1 MB).
`sizeLimitBytesWarning`	integer (bytes)	`512000` (500 KB)	Resources at or above this size, and below `sizeLimitBytesFail`, produce a warning. Must be lower than `sizeLimitBytesFail`.

An invalid value (missing, zero, negative, fractional) makes the auditor fall back to that preference’s default and log a warning. A warning limit equal to or above the fail limit is not an invalid value: it makes the audit fail until the thresholds are fixed.

What it audits

Every row maps to one checkRule* method and one CheckCode, in the order the auditor runs them.

CheckCode	Status	When it fires	Recommendation	Why it matters
`37A-LargeResourceAuditor-401`	`FAIL`	`sizeLimitBytesWarning` is equal to or above `sizeLimitBytesFail`. Message: `sizeLimitBytesWarning ({warning}) must be lower than sizeLimitBytesFail ({fail}).`	On the auditor’s Configuration tab, set `sizeLimitBytesWarning` below `{fail}`, or raise `sizeLimitBytesFail` above `{warning}`.	Until the warning threshold is lower than the fail threshold this audit cannot measure the page, so oversized files that slow it down go unreported.
`37A-LargeResourceAuditor-500`	`ERROR`	The page could not be loaded in the browser (browser error, timeout, or an HTTP error status other than a block). Message: `Could not load {url} in the browser: {error}.`	Make sure `{url}` loads in a browser (it answered: `{error}`), then run the audit again.	—
`37A-LargeResourceAuditor-100`	`INFORMATION`	Neither `HEAD` (`Content-Length`) nor `GET` could size a resource. Message: `Could not measure the size of {resource}: {problem}.`	Make sure `{resource}` answers a HEAD or GET request within 5 seconds; until then its size is not compared with the `{warning}` warning threshold.	—
`37A-LargeResourceAuditor-400`	`FAIL`	A resource is at or above `sizeLimitBytesFail`. Message: `Resource {resource} is {size}, at or above the {fail} fail threshold.`	Compress, resize or lazy-load `{resource}` (`{size}`) to bring it below `{fail}`, or raise `sizeLimitBytesFail` if the size is intentional.	Oversized files delay rendering and waste mobile data, so visitors wait longer and more of them leave before the page loads.
`37A-LargeResourceAuditor-300`	`WARNING`	A resource is at or above `sizeLimitBytesWarning` and below `sizeLimitBytesFail`. Message: `Resource {resource} is {size}, at or above the {warning} warning threshold and below the {fail} fail threshold.`	Compress or lazy-load `{resource}` (`{size}`) before it grows past `{fail}`.	Large files add load time on every visit, and visitors on slow mobile connections feel it most.
`37A-LargeResourceAuditor-200`	`SUCCESS`	All rules passed. Message: `All {n} measured resources are below {warning}; the largest is {resource} at {size}.`	—	—
`37A-LargeResourceAuditor-590`	`ERROR`	The audited page answered HTTP 403, HTTP 429 or a Cloudflare challenge, so 37AuditsBot was blocked before it could read the page.	Allow 37AuditsBot in your firewall or bot protection: allowlist its user agent, or verify its Web Bot Auth signature. See https://www.37audits.com/bot.	While 37AuditsBot is blocked this audit cannot look at the site, so problems on it go unreported.

Sizes are shown in MB with two decimals (1.20 MB); a size measured by a download that stopped at the fail limit reads at least 0.98 MB.

Output documentation

Status	Description	Test logic
INFORMATION	A resource could not be measured	Neither `HEAD` nor `GET` gave its size. Never changes the overall status.
SUCCESS	No resource reaches the warning limit	Every measured resource is below `sizeLimitBytesWarning`.
WARNING	One or more resources are large, none too large	At least one size is greater than or equal to `sizeLimitBytesWarning`, and none reaches `sizeLimitBytesFail`; the summary message counts them.
FAIL	One or more resources are too large, or the thresholds are inconsistent	At least one size is greater than or equal to `sizeLimitBytesFail` (the summary counts the failures and, when present, the warnings), or `sizeLimitBytesWarning` is not lower than `sizeLimitBytesFail`.
ERROR	The audit could not run	The page could not be loaded (`…-500`) or the site blocked 37AuditsBot (`…-590`).

Risks and considerations

Large assets hurt Core Web Vitals

Oversized hero images and videos are the most common cause of a slow Largest Contentful Paint.
Big JavaScript bundles block the main thread and delay interactivity on low-end devices.

What this auditor does not check

Resources loaded dynamically after the load event (for example by scripts or CSS url() references) are not discovered.
A resource behind a slow server (more than 5 seconds per request) is reported as unmeasured rather than compared with the limits.
The limits apply to every resource type alike; they do not distinguish a 1 MB video from a 1 MB script.

Auditors

MCP Server

REST API

Large Resource Auditor

Performance

Configurable

How it works

Preferences

What it audits

Output documentation

Risks and considerations

Large assets hurt Core Web Vitals

What this auditor does not check

Performance

Configurable

​How it works

​Preferences

​What it audits

​Output documentation

​Risks and considerations

​Large assets hurt Core Web Vitals

​What this auditor does not check

How it works

Preferences

What it audits

Output documentation

Risks and considerations

Large assets hurt Core Web Vitals

What this auditor does not check