🔥 Why Most React Apps are DOOMED To Lag — And How To Fix It With useDeferredValue

🔥 Why Most React Apps are DOOMED To Lag — And How To Fix It With useDeferredValue

If your React app starts lagging as soon as users interact with large lists, search inputs, or dynamic UIs — you’re not alone.

There’s a good chance you’ve run into unnecessary re-renders, high CPU usage, or the dreaded 😱 "Why is my simple dropdown lagging on scroll?!" performance bottlenecks.

Today, we’re going to talk about an underutilized and insanely powerful hook introduced in React 18 ––– useDeferredValue ––– and how it can completely transform your app's perceived speed with minimal changes.

🚨 The Problem: Too Many Fast Updates

Let’s say you have a large filtered list. A user types something in an input box and expects filtered results to update accordingly. Pretty basic, right?

Here's a naive implementation:

function SearchList({ data }) {
  const [search, setSearch] = React.useState("");

  const filtered = data.filter((item) =>
    item.toLowerCase().includes(search.toLowerCase())
  );

  return (
    <div>
      <input
        type="text"
        value={search}
        onChange={(e) => setSearch(e.target.value)}
      />
      <ul>
        {filtered.map((item) => (
          <li key={item}>{item}</li>
        ))}
      </ul>
    </div>
  );
}

Why this causes lag:

• Every keystroke immediately forces the list to re-render.
• If the list is long (~1000+ items), this adds heavy load to the UI thread.
• The result? Input lags, the UI locks up, users rage quit. 🔥

✅ The React 18 Fix: useDeferredValue

React 18 introduced Concurrent Features, and one of the gems is useDeferredValue. It tells React:

"Hey, this value update isn't urgent. Prioritize input, rendering can happen later."

Let’s use it:

import { useState, useDeferredValue } from "react";

function SearchList({ data }) {
  const [search, setSearch] = useState("");

  // This value will lag behind the actual input but improve UI responsiveness
  const deferredSearch = useDeferredValue(search);

  const filtered = data.filter((item) =>
    item.toLowerCase().includes(deferredSearch.toLowerCase())
  );

  return (
    <div>
      <input
        type="text"
        value={search}
        onChange={(e) => setSearch(e.target.value)}
      />
      <ul>
        {filtered.map((item) => (
          <li key={item}>{item}</li>
        ))}
      </ul>
    </div>
  );
}

Now the user can type instantly and fluently, while the filtered list updates in the background. Huge win for perceived performance.

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🤯 Bonus: Show Loading States

Since deferredSearch trails behind the actual search, you can now detect when a render is pending 🧠

import { useTransition } from 'react';

function SearchList({ data }) {
  const [search, setSearch] = useState("");
  const deferredSearch = useDeferredValue(search);

  const isPending = search !== deferredSearch;

  const filtered = data.filter((item) =>
    item.toLowerCase().includes(deferredSearch.toLowerCase())
  );

  return (
    <div>
      <input
        type="text"
        value={search}
        onChange={(e) => setSearch(e.target.value)}
      />
      {isPending && <p>Updating results...</p>}
      <ul>
        {filtered.map((item) => (
          <li key={item}>{item}</li>
        ))}
      </ul>
    </div>
  );
}

Your app now has real-time feedback and zero perceived jank.

🧪 Use Case: Large Data Sets

Let’s test it with 10,000 items.

const data = Array.from({ length: 10000 }, (_, i) => `Item ${i + 1}`); // 10,000 items

Use this data with and without useDeferredValue. You’ll immediately notice that typing into the input is buttery smooth with the hook in place and painfully choppy without it.

⚠️ Important Notes

• useDeferredValue does not magically solve all performance issues.
• Works best for delaying expensive UI updates without impacting user interaction.
• Combine with virtualization (e.g., react-window) for further optimization.
• Needs React 18+. Not supported in earlier versions.

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🧠 React Performance is UX

Performance isn’t about milliseconds — it’s about user frustration. A few dropped frames can kill your app's joy. useDeferredValue is one of those rare, simple tools that feels like cheating.

Try it today. Your users will thank you.

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💡 TL;DR

• Laggy UI? Consider useDeferredValue to prioritize user input.
• Great for large lists, UI-heavy filters, and any text-input based re-rendering.
• Adds minimal code, but huge perceived performance boost.

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🛠️ Further Reading

• React Docs – useDeferredValue
• React 18 Concurrent Features
• Case Study: Instagram switch to Concurrent Mode

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⏱ Don't wait. Go refactor that janky search input now!

💡 If you need React performance optimization or frontend expertise — we offer Frontend Development Services.