Add major performance breakthroughs guide for React on Rails Pro v4 and React on Rails v15

Author: AbanoubGhadban

docs/react-on-rails-pro/major-performance-breakthroughs-upgrade-guide.md

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+# React on Rails Pro: Major Performance Breakthroughs - React Server Components, SSR Streaming & Early Hydration
+
+**Subject: 🚀 Revolutionary Performance Breakthroughs: React Server Components, SSR Streaming & Early Hydration Now Available in React on Rails Pro v4 & React on Rails v15**
+
+---
+
+June 24, 2025
+
+We're thrilled to announce a major update: [React on Rails Pro](https://www.shakacode.com/react-on-rails-pro/) v4 and [React on Rails](https://github.com/shakacode/react_on_rails) v15 now deliver **unprecedented performance improvements** that will transform your applications. These updates introduce multiple breakthrough technologies that work together to deliver the fastest possible user experience.
+
+## 🎯 What This Means for Your Applications
+
+- **Dramatically faster load times**
+- **Smaller JavaScript bundles**
+- **Better Core Web Vitals**
+- **Improved SEO**
+- **Smoother user interactions**
+- **Eliminated race conditions**
+- **Optimized streaming performance**
+
+## 🔥 React Server Components
+
+Server Components execute on the server and stream HTML to the client—no server-side JavaScript in your bundle. Real‑world results include:
+
+- [productonboarding.com experiment](https://frigade.com/blog/bundle-size-reduction-with-rsc-and-frigade):
+  - **62% reduction** in client‑side bundle size
+  - **63% improvement** in Google Speed Index
+  - Total blocking time: **from 110 ms to 1 ms**
+- [geekyants.com Case Study](https://geekyants.com/en-gb/blog/boosting-performance-with-nextjs-and-react-server-components-a-geekyantscom-case-study):
+  - **52% smaller** JavaScript and TypeScript codebase
+  - Lighthouse scores improved **from ~50 to ~90**
+- [Airbnb's RSC migration results](<https://questlab.pro/blog-posts/web-development/wd-pl-2024-articleId912i1h212818#:~:text=%22Our%20migration%20to%20React%20Server%20Components%20resulted%20in%20a%2015%25%20improvement%20in%20core%20web%20vitals%20and%20a%2023%25%20reduction%20in%20Time%20to%20First%20Byte%20(TTFB)%20across%20all%20markets.%22%20%2D%20Airbnb%20Engineering%20Team>):
+- **15% improvement** in core web vitals
+- **23% reduction** in Time to First Byte
+- [Meta's developer portal migration](https://questlab.pro/blog-posts/web-development/wd-pl-2024-articleId912i1h212818#:~:text=Meta%27s%20RSC%20Implementation%20Results):
+  - **30% reduction** in JavaScript bundle size
+  - **60% improvement** in Time to Interactive
+  - **45% faster** First Contentful Paint
+  - **50% reduction** in server response time
+  - **25% decrease** in overall maintenance complexity
+
+Please note that only the first of these directly compares performance of equivalent applications with and without React Server Components.
+Other migrations may include React or other dependency upgrades and so on.
+
+## 🌊 SSR Streaming
+
+SSR Streaming sends HTML to the browser in chunks as it's generated, enabling progressive rendering:
+
+- [An experiment at Nordnet comparing equivalent applications with and without streaming SSR](https://www.diva-portal.org/smash/get/diva2:1903931/FULLTEXT01.pdf):
+  - **32% faster** time to first byte
+  - **40% faster** total blocking time
+  - Negative result: **2% increase** in server load
+- [Hulu case study](https://www.compilenrun.com/docs/framework/nextjs/nextjs-ecosystem/nextjs-case-studies/#case-study-3-hulus-streaming-platform):
+  - **30% faster** page load times
+- [styled‑components v3.1.0: A massive performance boost and streaming server-side rendering support](https://medium.com/styled-components/v3-1-0-such-perf-wow-many-streams-c45c434dbd03)
+
+## ⚡️ **BREAKTHROUGH: Early Hydration Technology**
+
+**React on Rails now starts hydration even before the full page is loaded!** This revolutionary change delivers significant performance improvements across all pages:
+
+- **Eliminates Race Conditions**: No more waiting for full page load before hydration begins
+- **Faster Time-to-Interactive**: Components hydrate as soon as their server-rendered HTML reaches the client
+- **Streaming HTML Optimization**: Perfect for modern streaming responses - components hydrate in parallel with page streaming
+- **Async Script Safety**: Can use `async` scripts without fear of race conditions
+- **No More Defer Needed**: The previous need for `defer` to prevent race conditions has been eliminated
+
+This optimization is particularly impactful for:
+- **Streamed pages** where content loads progressively
+- **Large pages** with many components
+- **Slow network conditions** where every millisecond counts
+- **Modern web apps** requiring fast interactivity
+
+*Performance improvement visualization:*
+
+![Performance comparison showing early hydration improvement](https://github.com/user-attachments/assets/ae33fe14-42f1-4cc1-bde3-9c5bb570cdbf)
+
+*The image above demonstrates the dramatic performance improvement:*
+- **Left (Before)**: Hydration didn't start until the full page load completed, causing a huge delay before hydration
+- **Right (After)**: Hydration starts immediately as soon as components are available, without waiting for full page load
+- **Result**: Components now become interactive much faster, eliminating the previous race condition delays
+
+## 🚀 Enhanced Performance Infrastructure
+
+### Fastify-Based Node Renderer
+- **3x faster** Node renderer based on Fastify instead of Express
+- **HTTP/2 Cleartext** communication between Rails and Node renderer
+- **Multiplexing and connection reuse** for significantly better performance
+- **No code changes required** - automatic performance boost
+
+### Optimized Script Loading Strategies
+- New `generated_component_packs_loading_strategy` configuration
+- **Async loading by default** for Shakapacker ≥ 8.2.0 (optimal performance)
+- **Smart hydration timing** that works perfectly with streaming HTML
+- **Eliminated waterfall delays** in component hydration
+
+## 💰 Why This Upgrade is Critical
+
+These performance improvements aren't just nice-to-haves—they're essential for:
+
+- **Competitive advantage** in today's performance-focused web landscape
+- **SEO improvements** as Core Web Vitals become ranking factors
+- **User retention** - faster sites keep users engaged longer
+- **Conversion rates** - every millisecond counts for e-commerce
+- **Mobile performance** - crucial for global markets with slower connections
+
+---
+
+Adopting these features in React on Rails Pro v4 and React on Rails v15 will help you deliver **dramatically faster, leaner, and more SEO‑friendly applications** with fewer client‑side resources and eliminated performance bottlenecks.
+
+**Ready to get started?**
+
+1. Update to React on Rails v15
+2. Update to React on Rails Pro v4
+3. Follow our [RSC & SSR Streaming migration guide](https://www.shakacode.com/react-on-rails-pro/docs/react-server-components/tutorial/)
+
+Let's make your apps faster—together.
+
+**ShakaCode Team**
+*Building the future of Rails + React performance*