React Hydration & Browser Scheduling API: A Game Changer for Performance

The Problem: React Feels Slow Even After Page Load We’ve all experienced this—React feels slow even after the page loads. You see the UI, but you can’t interact with it. An input field is right there, but typing doesn’t work. Why? The answer lies in React’s Scheduler — React, not the browser, decides execution order. This leads to situations where UI elements are visible but unresponsive. Until now, developers have only had hacks to manage this issue, but now, there’s a real solution. To understand this problem in-depth, we first need to explore how React’s Scheduler works and why it causes hydration issues. How React Scheduler Works React uses a cooperative scheduling model to balance rendering tasks efficiently without blocking the UI. Instead of rendering everything synchronously, React breaks the rendering work into chunks, prioritizing tasks dynamically. 1. Task Prioritization with Fibers React’s Fiber architecture allows it to pause, prioritize, and resume rendering tasks. Tasks are categorized into priority levels: Immediate (Highest Priority): e.g., User interactions (clicks, keypress) User-blocking: e.g., Typing in an input field Normal: Default updates (e.g., state changes, re-renders) Low-priority: Background tasks (e.g., data prefetching) Idle (Lowest Priority): Can be deferred indefinitely 2. Scheduling Mechanism in React React used requestIdleCallback initially, but it was unpredictable across different browsers. Instead, React now uses MessageChannel to schedule tasks in the microtask queue, improving performance. You can read these below blogs, if you want to know scheduling in best way

Feb 23, 2025 - 17:20

React Hydration & Browser Scheduling API: A Game Changer for Performance

The Problem: React Feels Slow Even After Page Load

We’ve all experienced this—React feels slow even after the page loads. You see the UI, but you can’t interact with it. An input field is right there, but typing doesn’t work. Why?

The answer lies in React’s Scheduler — React, not the browser, decides execution order. This leads to situations where UI elements are visible but unresponsive. Until now, developers have only had hacks to manage this issue, but now, there’s a real solution.

To understand this problem in-depth, we first need to explore how React’s Scheduler works and why it causes hydration issues.

How React Scheduler Works

React uses a cooperative scheduling model to balance rendering tasks efficiently without blocking the UI. Instead of rendering everything synchronously, React breaks the rendering work into chunks, prioritizing tasks dynamically.

1. Task Prioritization with Fibers

React’s Fiber architecture allows it to pause, prioritize, and resume rendering tasks. Tasks are categorized into priority levels:

Immediate (Highest Priority): e.g., User interactions (clicks, keypress)
User-blocking: e.g., Typing in an input field
Normal: Default updates (e.g., state changes, re-renders)
Low-priority: Background tasks (e.g., data prefetching)
Idle (Lowest Priority): Can be deferred indefinitely

2. Scheduling Mechanism in React

React used requestIdleCallback initially, but it was unpredictable across different browsers. Instead, React now uses MessageChannel to schedule tasks in the microtask queue, improving performance.

You can read these below blogs, if you want to know scheduling in best way