core(lcp-lazy-loaded): add LCP savings estimate

core/audits/lcp-lazy-loaded.js

@@ -6,6 +6,8 @@
 
 import {Audit} from './audit.js';
 import * as i18n from '../lib/i18n/i18n.js';
+import {LCPBreakdown} from '../computed/metrics/lcp-breakdown.js';
+import {LargestContentfulPaint} from '../computed/metrics/largest-contentful-paint.js';
 
 const UIStrings = {
   /** Title of a Lighthouse audit that provides detail on whether the largest above-the-fold image was loaded with sufficient priority. This descriptive title is shown to users when the image was loaded properly. */
@@ -18,6 +20,8 @@ const UIStrings = {
 
 const str_ = i18n.createIcuMessageFn(import.meta.url, UIStrings);
 
+const ESTIMATED_PERCENT_SAVINGS = 0.15;
+
 class LargestContentfulPaintLazyLoaded extends Audit {
   /**
    * @return {LH.Audit.Meta}
@@ -29,7 +33,8 @@ class LargestContentfulPaintLazyLoaded extends Audit {
       failureTitle: str_(UIStrings.failureTitle),
       description: str_(UIStrings.description),
       supportedModes: ['navigation'],
-      requiredArtifacts: ['TraceElements', 'ViewportDimensions', 'ImageElements'],
+      requiredArtifacts: ['TraceElements', 'ViewportDimensions', 'ImageElements',
+        'traces', 'devtoolsLogs', 'GatherContext', 'URL'],
     };
   }
 
@@ -46,9 +51,10 @@ class LargestContentfulPaintLazyLoaded extends Audit {
 
   /**
    * @param {LH.Artifacts} artifacts
-   * @return {LH.Audit.Product}
+   * @param {LH.Audit.Context} context
+   * @return {Promise<LH.Audit.Product>}
    */
-  static audit(artifacts) {
+  static async audit(artifacts, context) {
     const lcpElement = artifacts.TraceElements.find(element => {
       return element.traceEventType === 'largest-contentful-paint' && element.type === 'image';
     });
@@ -59,7 +65,11 @@ class LargestContentfulPaintLazyLoaded extends Audit {
 
     if (!lcpElementImage ||
       !this.isImageInViewport(lcpElementImage, artifacts.ViewportDimensions)) {
-      return {score: null, notApplicable: true};
+      return {
+        score: null,
+        notApplicable: true,
+        metricSavings: {LCP: 0},
+      };
     }
 
     /** @type {LH.Audit.Details.Table['headings']} */
@@ -73,8 +83,27 @@ class LargestContentfulPaintLazyLoaded extends Audit {
       },
     ]);
 
+    const wasLazyLoaded = lcpElementImage.loading === 'lazy';
+
+    const metricComputationData = Audit.makeMetricComputationDataInput(artifacts, context);
+    const {timing: metricLcp} =
+      await LargestContentfulPaint.request(metricComputationData, context);
+    const lcpBreakdown = await LCPBreakdown.request(metricComputationData, context);
+    let lcpSavings = 0;
+    if (wasLazyLoaded && lcpBreakdown.loadStart !== undefined) {
+      // Estimate the LCP savings using a statistical percentage.
+      // https://web.dev/lcp-lazy-loading/#causal-performance
+      //
+      // LCP savings will be at most the LCP load delay.
+      const lcpLoadDelay = lcpBreakdown.loadStart - lcpBreakdown.ttfb;
+      lcpSavings = Math.min(metricLcp * ESTIMATED_PERCENT_SAVINGS, lcpLoadDelay);
+    }
+
     return {
-      score: lcpElementImage.loading === 'lazy' ? 0 : 1,
+      score: wasLazyLoaded ? 0 : 1,
+      metricSavings: {
+        LCP: lcpSavings,
+      },
       details,
     };
   }

core/test/audits/lcp-lazy-loaded-test.js

@@ -5,13 +5,18 @@
  */
 
 import LargestContentfulPaintLazyLoaded from '../../audits/lcp-lazy-loaded.js';
+import {defaultSettings} from '../../config/constants.js';
+import {createTestTrace, rootFrame} from '../create-test-trace.js';
+import {networkRecordsToDevtoolsLog} from '../network-records-to-devtools-log.js';
 
 const SAMPLE_NODE = {
   devtoolsNodePath: '1,HTML,1,BODY,3,DIV,2,IMG',
   selector: 'div.l-header > div.chorus-emc__content',
   nodeLabel: 'My Test Label',
   snippet: '<img class="test-class">',
 };
+const mainDocumentUrl = 'http://www.example.com';
+
 function generateImage(loading, clientRectTop) {
   return {
     src: 'test',
@@ -25,6 +30,7 @@ function generateImage(loading, clientRectTop) {
     node: SAMPLE_NODE,
   };
 }
+
 describe('Performance: lcp-lazy-loaded audit', () => {
   it('correctly surfaces the lazy loaded LCP element', async () => {
     const artifacts = {
@@ -40,10 +46,52 @@ describe('Performance: lcp-lazy-loaded audit', () => {
         innerHeight: 500,
         innerWidth: 300,
       },
+      traces: {
+        defaultPass: createTestTrace({
+          largestContentfulPaint: 1000,
+          topLevelTasks: [{ts: 10, duration: 1000}],
+        }),
+      },
+      devtoolsLogs: {
+        defaultPass: networkRecordsToDevtoolsLog([
+          {
+            url: mainDocumentUrl,
+            priority: 'High',
+            networkRequestTime: 100,
+            networkEndTime: 200,
+            timing: {sendEnd: 0},
+            frameId: rootFrame,
+          },
+          {
+            url: 'http://www.example.com/image.png',
+            priority: 'Low',
+            resourceType: 'Image',
+            networkRequestTime: 800,
+            networkEndTime: 900,
+            timing: {sendEnd: 0},
+            frameId: rootFrame,
+          },
+        ]),
+      },
+      URL: {
+        requestedUrl: mainDocumentUrl,
+        mainDocumentUrl,
+        finalDisplayedUrl: mainDocumentUrl,
+      },
+      GatherContext: {gatherMode: 'navigation'},
     };
 
-    const auditResult = await LargestContentfulPaintLazyLoaded.audit(artifacts);
+    const settings = JSON.parse(JSON.stringify(defaultSettings));
+    settings.throttlingMethod = 'devtools';
+
+    const context = {
+      computedCache: new Map(),
+      settings,
+    };
+
+    const auditResult = await LargestContentfulPaintLazyLoaded.audit(artifacts, context);
     expect(auditResult.score).toEqual(0);
+    expect(auditResult.metricSavings).toEqual({LCP: 150});
     expect(auditResult.details.items).toHaveLength(1);
     expect(auditResult.details.items[0].node.path).toEqual('1,HTML,1,BODY,3,DIV,2,IMG');
     expect(auditResult.details.items[0].node.nodeLabel).toEqual('My Test Label');
@@ -64,9 +112,52 @@ describe('Performance: lcp-lazy-loaded audit', () => {
         innerHeight: 500,
         innerWidth: 300,
       },
+      traces: {
+        defaultPass: createTestTrace({
+          largestContentfulPaint: 1000,
+          topLevelTasks: [{ts: 10, duration: 1000}],
+        }),
+      },
+      devtoolsLogs: {
+        defaultPass: networkRecordsToDevtoolsLog([
+          {
+            url: mainDocumentUrl,
+            priority: 'High',
+            networkRequestTime: 100,
+            networkEndTime: 200,
+            timing: {sendEnd: 0},
+            frameId: rootFrame,
+          },
+          {
+            url: 'http://www.example.com/image.png',
+            priority: 'Low',
+            resourceType: 'Image',
+            networkRequestTime: 800,
+            networkEndTime: 900,
+            timing: {sendEnd: 0},
+            frameId: rootFrame,
+          },
+        ]),
+      },
+      URL: {
+        requestedUrl: mainDocumentUrl,
+        mainDocumentUrl,
+        finalDisplayedUrl: mainDocumentUrl,
+      },
+      GatherContext: {gatherMode: 'navigation'},
     };
-    const auditResult = await LargestContentfulPaintLazyLoaded.audit(artifacts);
+
+    const settings = JSON.parse(JSON.stringify(defaultSettings));
+    settings.throttlingMethod = 'devtools';
+
+    const context = {
+      computedCache: new Map(),
+      settings,
+    };
+
+    const auditResult = await LargestContentfulPaintLazyLoaded.audit(artifacts, context);
     expect(auditResult.score).toEqual(1);
+    expect(auditResult.metricSavings).toEqual({LCP: 0});
     expect(auditResult.details.items).toHaveLength(1);
   });
 
@@ -86,6 +177,7 @@ describe('Performance: lcp-lazy-loaded audit', () => {
       },
     };
     const auditResult = await LargestContentfulPaintLazyLoaded.audit(artifacts);
+    expect(auditResult.metricSavings).toEqual({LCP: 0});
     expect(auditResult.notApplicable).toEqual(true);
   });
 
@@ -97,6 +189,7 @@ describe('Performance: lcp-lazy-loaded audit', () => {
 
     const auditResult = await LargestContentfulPaintLazyLoaded.audit(artifacts);
     expect(auditResult.score).toEqual(null);
+    expect(auditResult.metricSavings).toEqual({LCP: 0});
     expect(auditResult.notApplicable).toEqual(true);
   });
 
@@ -119,6 +212,9 @@ describe('Performance: lcp-lazy-loaded audit', () => {
     expect(auditResult).toEqual({
       score: null,
       notApplicable: true,
+      metricSavings: {
+        LCP: 0,
+      },
     });
   });
 });