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core(render-blocking-resources): reduce metric savings if LCP is an image #15694

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Dec 20, 2023
Merged

core(render-blocking-resources): reduce metric savings if LCP is an image #15694

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f17acf8
reduce impact of lcp savings if lcp is image
adrianaixba Dec 14, 2023
b30d90b
add test
adrianaixba Dec 14, 2023
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24 changes: 15 additions & 9 deletions core/audits/byte-efficiency/render-blocking-resources.js
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,8 @@ import {NetworkRequest} from '../../lib/network-request.js';
import {ProcessedNavigation} from '../../computed/processed-navigation.js';
import {LoadSimulator} from '../../computed/load-simulator.js';
import {FirstContentfulPaint} from '../../computed/metrics/first-contentful-paint.js';
import {LCPImageRecord} from '../../computed/lcp-image-record.js';


/** @typedef {import('../../lib/dependency-graph/simulator/simulator').Simulator} Simulator */
/** @typedef {import('../../lib/dependency-graph/base-node.js').Node} Node */
Expand Down Expand Up @@ -125,7 +127,7 @@ class RenderBlockingResources extends Audit {
/**
* @param {LH.Artifacts} artifacts
* @param {LH.Audit.Context} context
* @return {Promise<{wastedMs: number, results: Array<{url: string, totalBytes: number, wastedMs: number}>}>}
* @return {Promise<{fcpWastedMs: number, lcpWastedMs: number, results: Array<{url: string, totalBytes: number, wastedMs: number}>}>}
*/
static async computeResults(artifacts, context) {
const gatherContext = artifacts.GatherContext;
Expand Down Expand Up @@ -179,18 +181,21 @@ class RenderBlockingResources extends Audit {
}

if (!results.length) {
return {results, wastedMs: 0};
return {results, fcpWastedMs: 0, lcpWastedMs: 0};
}

const wastedMs = RenderBlockingResources.estimateSavingsWithGraphs(
const fcpWastedMs = RenderBlockingResources.estimateSavingsWithGraphs(
simulator,
fcpSimulation.optimisticGraph,
deferredNodeIds,
wastedCssBytes,
artifacts.Stacks
);

return {results, wastedMs};
const lcpRecord = await LCPImageRecord.request(metricComputationData, context);

// In most cases if the LCP is an image, render blocking resources don't affect LCP. For these cases we should reduce its impact.
return {results, fcpWastedMs, lcpWastedMs: lcpRecord ? 0 : fcpWastedMs};
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}

/**
Expand Down Expand Up @@ -276,11 +281,12 @@ class RenderBlockingResources extends Audit {
* @return {Promise<LH.Audit.Product>}
*/
static async audit(artifacts, context) {
const {results, wastedMs} = await RenderBlockingResources.computeResults(artifacts, context);
const {results, fcpWastedMs, lcpWastedMs} =
await RenderBlockingResources.computeResults(artifacts, context);

let displayValue;
if (results.length > 0) {
displayValue = str_(i18n.UIStrings.displayValueMsSavings, {wastedMs});
displayValue = str_(i18n.UIStrings.displayValueMsSavings, {wastedMs: fcpWastedMs});
}

/** @type {LH.Audit.Details.Opportunity['headings']} */
Expand All @@ -291,15 +297,15 @@ class RenderBlockingResources extends Audit {
];

const details = Audit.makeOpportunityDetails(headings, results,
{overallSavingsMs: wastedMs});
{overallSavingsMs: fcpWastedMs});

return {
displayValue,
score: results.length ? 0 : 1,
numericValue: wastedMs,
numericValue: fcpWastedMs,
numericUnit: 'millisecond',
details,
metricSavings: {FCP: wastedMs, LCP: wastedMs},
metricSavings: {FCP: fcpWastedMs, LCP: lcpWastedMs},
};
}
}
Expand Down
2 changes: 1 addition & 1 deletion core/test/audits/byte-efficiency/render-blocking-resources-test.js
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Original file line number Diff line number Diff line change
Expand Up @@ -88,7 +88,7 @@ describe('Render blocking resources audit', () => {
// it look like Montserrat starts after Fira Sans finishes. It would be preferred
// if eventual simulation improvements list Montserrat here as well.
]);
expect(result.metricSavings).toEqual({FCP: 469, LCP: 469});
expect(result.metricSavings).toEqual({FCP: 469, LCP: 0});
});

describe('#estimateSavingsWithGraphs', () => {
Expand Down
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