How to debug Interaction to Next Paint (INP) in Chrome
Jul 15, 2025 am 12:39 AMThe core of debugging INP is to locate page interaction delay issues. Use the Performance panel to record interactions and view time-consuming events; check Long Tasks and main thread blocking, split time-consuming logic or use Web Worker; analyze event listeners, reduce synchronous operations and execute asynchronously; use Lighthouse to identify problem areas such as long tasks and main thread time; combine tools to gradually troubleshoot and optimize to improve INP performance.
Debugging Interaction to Next Paint (INP) is actually the core of finding out the latency problem during page interaction. Chrome provides a number of tools and methods to help us locate the key bottlenecks that affect INP. The key is to find the specific reasons why the page responds slowly after user operations (such as clicks, inputs, etc.).

The following starts from several common angles to help you troubleshoot and optimize step by step:
1. Use the Performance panel to record user interactions
Open Chrome DevTools, switch to the Performance tab, click the Record button in the upper left corner, and manually simulate a user interaction (such as clicking a button or input box). After stopping recording, you will see a timeline.

- Find the Interactions section in the Summary area to see the time-consuming of each interactive event.
- Find the long-term interaction, click on it to see the call stack and main thread blocking.
Note: INP focuses mainly on the "worst" interaction, so even if most interactions are fast, as long as one is particularly long, the INP will be very poor.
2. Check for Long Tasks and main thread blocking
INP is greatly affected by the main thread's tasks. If there are long-running tasks during user operations (Long Tasks), the page will not respond in a timely manner.

- Find a red-marked Long Task in the Performance Timeline.
- Click on the task details to see which JavaScript is taking too much time, such as:
- Large data processing
- DOM operation frequently
- Third-party script blocking
Suggested practices:
- Split the time-consuming logic into small pieces and execute asynchronously using
setTimeout
orrequestIdleCallback
. - Use Web Worker to handle complex calculations.
- Lazy loading of non-critical function code.
3. Analyze event listeners and callback functions
Many INP problems are caused by the event listener itself being executed for too long. For example, after clicking a button, the bound callback function does too many synchronization operations.
- Find the Call Tree after the corresponding event is triggered in the Performance panel.
- Check if there is a function that has been executed for more than 100ms.
- Pay special attention to whether unnecessary DOM updates or API requests are made during event processing.
Optimization ideas:
- Reduces synchronization operations in event callbacks.
- Use debounce or throttle to control the frequency of high-frequency event.
- Move some operations to micro-tasks or macro-tasks to execute asynchronously.
4. Quickly identify problem areas with Lighthouse
In addition to manual analysis, Lighthouse can also help you quickly discover potential INP problems.
- Open DevTools → Lighthouse tab → check "Performance" and run the test.
- Check out the "Diagnostics" section of the report, which may prompt:
- Number of long tasks
- Main thread working time
- The issue of event listener taking too long
Although Lighthouse cannot directly tell you which line of code causes INP, it can point out the direction of optimization.
Basically that's it. Debugging INP is not too difficult, but it is easy to ignore details, such as a large-scale synchronized calculation after a button is clicked, or a third-party library slows down the response speed. Combining Performance and Lighthouse more, you can improve by gradual investigation.
The above is the detailed content of How to debug Interaction to Next Paint (INP) in Chrome. For more information, please follow other related articles on the PHP Chinese website!

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