The key to front-end performance analysis is to locate bottlenecks and use browser developer tools to view core metrics. First, use the Performance panel to analyze the execution time of FP, FCP, LCP, CLS and JS; second, optimize the resource loading order through the Network panel, compress pictures, merge CSS/JS, and set cache heads; second, monitor user interaction and long tasks, move complex calculations to Web Worker, and throttling and anti-shaking for high-frequency events; finally, introduce the performance monitoring SDK for automated reporting, and continuously optimize the experience.
Front-end performance analysis is actually not difficult, but many people tend to ignore key points at the beginning. Simply put, it is to find out the root causes of problems such as slow page loading and interaction lag through tools and methods. The focus is on positioning bottlenecks, not blindly optimizing.
Use browser developer tools to view core metrics
Chrome DevTools is the most commonly used front-end performance analysis tool, and the Performance panel can help you see the entire page loading process. The way to open is very simple: right-click the page and select "Check" → switch to the Performance tab → click the recording button in the upper left corner, and after refreshing the page, you can see the detailed record.
Focus on several indicators:
- FP (First Paint) and FCP (First Contentful Paint) : Time when the page first renders content
- LCP (Largest Contentful Paint) : Maximum content drawing time, affecting user experience perception
- CLS (Cumulative Layout Shift) : Page layout offset, affecting visual stability
- JS execution time : The main thread is occupied by JS for too long, which will cause the page to stutter.
You can also see the resource loading waterfall diagram in the panel. Which script or image slows down the overall loading is clear at a glance.
Analyze network requests and optimize loading order
Network panels are another important entry. It can show the time, size, and load order of each request. From it you can find large-sized files, repeated requests, or block rendering resources.
Frequently asked questions include:
- The picture is not compressed, it is hundreds of KB or even larger
- Multiple JS files are not merged, resulting in multiple requests
- Not using CDN or cache policy is unreasonable
- Render blocking resources (such as synchronous
<script></script>
)
Suggested practices:
- Add
srcset
andloading="lazy"
to the picture - Merge CSS/JS and use asynchronous loading of non-critical resources
- Setting up the appropriate HTTP cache header
- Reduce font file size with font subsets
Monitor user interaction and long tasks
Whether the user's operation is smooth depends largely on the execution efficiency of JavaScript. You can use the Main thread flame graph in the Performance panel to view the function call stack and execution time.
If you find that a function is executed for more than 50ms, it may cause interaction delay. You can consider this at this time:
- Put complex calculations into Web Worker to execute
- Throttle or anti-shake for high-frequency events (such as scrolling, input)
- Delay loading of non-essential functional modules
You can also use console.time()
and console.timeEnd()
to manually mark the execution time of a piece of code for easy debugging.
Automated monitoring and reporting performance data
It is not enough to complete the development environment analysis, and the experience of real online users is more important. A lightweight performance monitoring SDK can be introduced to automatically collect FP, FCP, FID, and CLS indicators and report to the server.
The implementation is usually to insert a small piece of JS script at the bottom of the page to listen for performance.timing
and PerformanceObserver
data. You can also use ready-made solutions, such as Google's Web Vitals library.
It should be noted that you do not over-collect to avoid affecting the user's performance itself.
Basically all this is it. Front-end performance analysis is not a one-time job, but a process of continuous optimization. The tools are not complicated, the key is to see more, try more, and compare more.
The above is the detailed content of How to profile frontend performance. For more information, please follow other related articles on the PHP Chinese website!

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