Simulated Lighthouse Report
Lighthouse Report for "Product Detail Page"
(Simulated Run, Mobile Device, Emulated Throttling)
Overall Score:
- Performance: 42/100
- Accessibility: 95/100 (Passed)
- Best Practices: 88/100 (Passed)
- SEO: 90/100 (Passed)
Core Web Vitals Assessment
- Largest Contentful Paint (LCP): 5.8s (Needs improvement)
- First Input Delay (FID): 350ms (Needs improvement)
- Cumulative Layout Shift (CLS): 0.31 (Needs improvement)
Metrics
- First Contentful Paint (FCP): 3.1s
- Speed Index (SI): 7.2s
- Total Blocking Time (TBT): 780ms
- Largest Contentful Paint (LCP): 5.8s
- Cumulative Layout Shift (CLS): 0.31
Opportunities (Potential Savings)
- Eliminate render-blocking resources: 2.5s
app.css(450 KB)vendor.js(890 KB)
- Serve images in next-gen formats: 850 KB
product-hero.jpg(2.1 MB)thumbnail-gallery-1.jpeg(650 KB)
- Efficiently encode images: 620 KB
product-hero.jpg(Original: 2.1 MB, Optimized: 1.4 MB)promo-banner.png(Original: 450 KB, Optimized: 280 KB)
- Reduce unused JavaScript: 550 KB
analytics.js(300 KB, 80% unused)polyfill.js(200 KB, 60% unused)
- Minimize main-thread work: 900ms
- Script Evaluation: 600ms
- Layout & Style: 250ms
- Avoid chaining critical requests: 1.5s
- Example: Font requests blocked by CSS, API calls blocking image loads.
- Preload key requests: 0.8s
- Missing preload for LCP image (
product-hero.jpg)
- Missing preload for LCP image (
Diagnostics
- Large Largest Contentful Paint image: The LCP element is a large product image (
product-hero.jpg) that is not optimized and loaded late. - Avoid long main-thread tasks: Several JavaScript tasks exceed 50ms, causing UI unresponsiveness.
- Excessive DOM size: The page has 3,500+ DOM nodes, impacting layout and style calculations.
- Third-party code heavily impacts main thread:
analytics.jsandads.jsconsume significant CPU time. - Avoid enormous network payloads: Total network payload is 7.5 MB.
- Ensure text remains visible during webfont load: Failed (FOIT issue).
Audits Passed
- Uses HTTPS
- All images have explicit width and height
- Avoids front-end JavaScript libraries with known security vulnerabilities
Audits Failed
- Avoids non-composited animations
- Images are not lazy-loaded
From the Lighthouse report, identify the top 3-4 primary performance bottlenecks. For each bottleneck, explain how it specifically impacts the Core Web Vitals (LCP, FID, CLS).
Based on your analysis of the Lighthouse report, prioritize 3-4 concrete optimization tasks. Justify your prioritization order, considering both impact (from 'Potential savings') and effort.
For the top 2 prioritized tasks from Question (2), provide specific technical solutions or strategies that a front-end developer would implement to address them.
How would you measure the impact of your optimizations and continuously monitor the page's performance after deployment? Name at least two tools or methods.
Beyond the specific findings in this report, what are two general web performance best practices that are crucial for e-commerce sites and why?