Main-thread work from the Lighthouse run. Reported when Lighthouse measured it; NOT_APPLICABLE on a static scan.
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How this rule is weighted
- Importance
- 3 / 10
- How much this matters relative to other rules.
- Confidence
- SPECIFICATION
- A standard says so: HTML, WCAG, an RFC or Google's own documentation.
- Severity
- warning
- How the finding is presented when it fails.
- Scoring
- Excluded
- Reported in the report, never penalised.
Impact is importance multiplied by the confidence tier’s weight — SPECIFICATION 1.0, RESEARCH 0.7, EMERGING 0.3, EXPERIMENTAL 0.1. Two rules backed by the same class of evidence therefore always carry the same weight, which is what makes the score reproducible rather than hand-tuned.
Questions about this rule
- Does PERF_LH_MAINTHREAD_WORK_BREAKDOWN affect my score?
- No. It is a detection rule: it runs, appears in your report and never penalises you. "Main-thread work within budget" is information about the page rather than a defect - most pages are not expected to satisfy it.
- What does SPECIFICATION confidence mean?
- A standard says so: HTML, WCAG, an RFC or Google's own documentation. Confidence is a declared tier rather than a per-rule number, so every rule backed by the same class of evidence carries the same weight - which is what makes the score reproducible instead of hand-tuned.
- How do I check PERF_LH_MAINTHREAD_WORK_BREAKDOWN on my own site?
- Paste your URL into the box above and it runs only this rule, usually in a couple of seconds. It is also included in the Performance checker and in a full scan.
Tags
- performance
- lighthouse
Related Performance rules
- Largest Contentful Paint within Core Web Vitals thresholdLCP measures perceived load speed; Google's "good" threshold is 2.5s.PERF_LCP
- Cumulative Layout Shift within Core Web Vitals thresholdCLS measures visual stability; Google's "good" threshold is 0.1.PERF_CLS
- First Contentful Paint within recommended thresholdFCP is when the user first sees anything at all. Beyond ~1.8s the page feels slow before any content has arrived.PERF_FCP
- Images declare width and heightAn <img> without intrinsic dimensions occupies zero height until it loads, then pushes everything below it down. Declaring width and height lets the browser reserve the space up front, which is the primary fix for layout shift.PERF_IMAGE_DIMENSIONS
- No render-blocking scripts in <head>A synchronous script in the head halts HTML parsing until it downloads and executes, delaying first paint by exactly that long.PERF_RENDER_BLOCKING_SCRIPTS
- Response is compressedServing HTML uncompressed typically transfers three to four times more bytes than necessary.PERF_COMPRESSION