Beyond the Virtual DOM: React’s Concurrent Powers for Smarter UI Rendering

React's Virtual DOM was once the cornerstone of its blazing-fast performance. But with the release of React 18, a shift has occurred. While the Virtual DOM still plays a role, it's no longer the crown jewel. Instead, concurrent rendering, server-side streaming, and server components are reshaping how we build modern UIs.

Let’s walk through what changed — and what it means for developers.

A Refresher: How Virtual DOM Works

The Virtual DOM (VDOM) is an in-memory representation of the actual DOM. Every time your state changes, React creates a new VDOM tree, compares it (diffing) with the previous one, and calculates the minimal set of operations needed to update the real DOM (reconciliation).

Example:

function App() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <h1>Clicked {count} times</h1>
      <button onClick={() => setCount(count + 1)}>Click me</button>
    </div>
  );
}

When setCount triggers, React:

1. Creates a new virtual DOM tree.
2. Compares it with the old one.
3. Applies only the changed part (h1 text) to the real DOM.

While efficient, this process becomes CPU-intensive with larger trees and more frequent updates.

Fiber Architecture: The First Crack in the VDOM Crown

The original Virtual DOM was fast — but blocking. All updates were processed in one go, which could freeze the UI for complex apps.

Fiber (introduced in React 16) was a reimplementation of React's core to solve this. Think of it as breaking rendering work into smaller units that can be paused, interrupted, or prioritized.

How Fiber Works:

• Splits work into chunks (called "fibers")
• Interruptible: Rendering can pause to let urgent tasks (like input) go first
• Prioritization: High-priority work (like clicks) is handled first, low-priority (like filtering a large list) can wait

Fiber laid the foundation for Concurrent Rendering, which was fully realized in React 18.

React 18: What Really Changed Under the Hood?

With React 18, rendering became asynchronous by default. React can now schedule work, pause it, and continue when the browser is free — thanks to its Concurrent Rendering engine.

This isn't about rendering faster — it’s about rendering smarter, based on what's urgent and what can wait.

Understanding Concurrent Updates

Before React 18, all updates were processed synchronously. If one update took time (like filtering a large list), it could block everything — even typing input.

Now, with Concurrent Updates, React can:

• Interrupt low-priority rendering
• Process urgent updates (like keystrokes) first
• Resume paused rendering when resources free up

This keeps the app responsive, even during heavy computations or re-renders.

This gives you new tools to build faster, more responsive UIs:

• startTransition() – Mark non-urgent updates so React can defer them
• useDeferredValue() – Defer rendering of heavy values to avoid blocking user interactions
• Streaming SSR – Send HTML in chunks as it’s generated on the server
• Server Components – Shift rendering logic to the server to reduce client-side JS

Together, these tools help you build UIs that remain fast and responsive — no matter how complex your app gets.

Example of startTransition()

import { useState, startTransition } from 'react';

function Search() {
  const [input, setInput] = useState('');
  const [results, setResults] = useState([]);

  const handleChange = (e) => {
    const value = e.target.value;
    setInput(value); // urgent: update input immediately

    // non-urgent: update results without blocking UI
    startTransition(() => {
      const filtered = someBigList.filter(item =>
        item.includes(value)
      );
      setResults(filtered);
    });
  };

  return <input value={input} onChange={handleChange} />;
}

What's happening: Typing feels smooth, even if someBigList is huge — because filtering runs in the background.

Example of useDefferedValue()

import { useState, useDeferredValue, useMemo } from 'react';

function SearchResults({ query }) {
  const deferredQuery = useDeferredValue(query); // lower priority

  // simulate expensive filtering
  const results = useMemo(() => {
    return bigList.filter(item =>
      item.includes(deferredQuery)
    );
  }, [deferredQuery]);

  return <div>{results.length} results found</div>;
}

What's happening: The UI updates instantly with the typed query, but filtering happens after — avoiding lag.

When Virtual DOM Still Makes Sense

• For local UI interactions like toggles, counters, and form inputs

• When using client-side routing (e.g., SPAs with animations)

• In smaller apps where SSR/streaming overhead isn't justified

• For existing codebases — migrating entirely isn’t necessary

The Virtual DOM is still powerful, just no longer the only option.

Developer Takeaways: Mindset Shifts in Component Design

With the shift introduced by React 18, developers need to evolve beyond the traditional Virtual DOM mindset. Previously, we relied on a simple flow — state changes trigger re-renders, and React optimizes the rest. Most of the logic lived on the client, and hooks were primarily used for managing UI state. But with Concurrent Rendering and server-first architecture, the focus has shifted. Now, it’s about prioritizing updates based on urgency, offloading as much work as possible to the server, and designing components with rendering budgets and user experience in mind. It’s no longer about just rendering quickly — it’s about rendering intentionally.

Conclusion: Balance, Not Abandonment

The Virtual DOM hasn’t disappeared — it’s just no longer the centerpiece of performance. React 18 promotes a hybrid rendering strategy where SSR, streaming, and concurrency take the lead — and the Virtual DOM plays a supporting role.

Render like it’s 2025: concurrent, responsive, and server-smart..
Happy Coding❤️