INP optimization: How to master the browser event loop

We’ve all been there: you ship a feature that works perfectly on your development machine, but as soon as it hits the wild, your Interaction to Next Paint (INP) scores plummet. Last month, I spent about three days chasing a sluggish "save" button that caused a full-page freeze whenever a user triggered a complex data validation routine.

The culprit wasn't a slow network or a massive DOM; it was a single, monolithic JavaScript function blocking the main thread for over 400ms. When the main thread is occupied with a long task, the browser’s event loop can't process user clicks or keystrokes, leading to a poor user experience. If you’re struggling with similar bottlenecks, you’re likely fighting the same event loop congestion.

Understanding the Event Loop and INP

The browser's main thread is a single-lane highway. When you fire off a heavy computation, you’re essentially parking a semi-truck in that lane. Everything else—painting, scrolling, and input handling—has to wait for that truck to finish its business.

INP optimization is the practice of ensuring that the highway is never blocked for long. Since the browser processes tasks in a queue, a long task prevents the event loop from picking up input events. To keep your app snappy, you need to break these tasks into smaller, manageable chunks that allow the browser to breathe between operations.

Breaking Down Tasks for INP Optimization

Before reaching for advanced APIs, the most effective strategy is simple task decomposition. Instead of executing a massive processData() function, we can break it into smaller segments using setTimeout(fn, 0) or scheduler.yield().

Here is a simple pattern I often use to chunk large arrays:

JAVASCRIPT
async function processLargeData(items) {
  for (const item of items) {
    // Perform a small slice of work
    performWork(item);

    // Yield control back to the browser
    if (shouldYield()) {
      await new Promise(resolve => setTimeout(resolve, 0));
    }
  }
}

This approach is effective, but it forces a yield every single time, which might be overkill if there's no pending input. This is where isInputPending becomes a game-changer.

Leveraging isInputPending

The isInputPending API allows us to ask the browser, "Hey, does the user need the main thread right now?" If the answer is yes, we can yield immediately. If not, we keep chugging along. It’s a surgical approach to task scheduling.

Here is how I implemented it in a recent project:

JAVASCRIPT
function processChunks(items) {
  let i = 0;
  function work() {
    const start = performance.now();
    
    while (i < items.length) {
      process(items[i++]);
      
      // Check for user input every 16ms
      if (navigator.scheduling.isInputPending() || performance.now() - start > 16) {
        setTimeout(work, 0);
        return;
      }
    }
  }
  work();
}

By checking isInputPending, we only yield when necessary. This keeps the UI responsive without sacrificing the throughput of our background tasks. It’s far more efficient than blindly yielding every few iterations.

Addressing Core Web Vitals Beyond the Loop

While managing the event loop is crucial, it’s rarely the only factor in your Core Web Vitals. You might also want to look into the Server-Timing API for INP Optimization: Debugging Backend Latency to ensure your backend isn't adding unnecessary delay.

Additionally, avoid the trap of "forced synchronous layout" where your JS reads and writes to the DOM repeatedly. If you suspect this is happening, Optimizing Forced Synchronous Layout: A Guide to Better INP is a great resource for identifying those silent killers.

If you’re dealing with heavy data, consider Data Hydration Strategies: Improving LCP and INP Performance to ensure that your initial load doesn't lock the thread while trying to process massive JSON blobs.

FAQ

Is isInputPending supported everywhere? It's currently a Chromium-only feature. You should always feature-detect it and provide a fallback that simply yields after a fixed time duration, like I showed in the code snippet above.

Does this replace Web Workers? Not exactly. Web Workers are still the gold standard for offloading heavy computation. However, isInputPending is excellent for tasks that must touch the DOM or need access to the current state of the page, where a Worker would be too cumbersome.

How do I know if my task decomposition is working? Use the Chrome DevTools "Performance" tab. Look for "Long Tasks" (red triangles). If you’ve successfully decomposed your tasks, those long blocks will shrink, and you’ll see more gaps where the browser can handle input events.

I’m still experimenting with how this interacts with modern frameworks like React 18, which has its own concurrent features. Sometimes, adding your own scheduling on top of framework internals can lead to unexpected behavior. If you’re finding that your manual scheduling fights with your framework's own task queue, you might be better off sticking to standard requestIdleCallback patterns for non-critical work, as discussed in requestIdleCallback and Main Thread Optimization for Smooth UIs.

Next time, I’m planning to test if scheduler.yield() becomes a more stable replacement for my setTimeout hacks once it lands in more browsers. For now, keep your tasks short, keep your users happy, and never trust a "fast enough" function without checking it in a profile.