Fastify + React vs Leptos

Real-world performance comparison based on identical SaaS dashboard implementations.

Fastify + React

Fastify + React

VPS

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Node.js high-performance web framework combined with React SSR. Fastify is one of the fastest Node.js frameworks, built on a robust plugin architecture with JSON Schema validation and automatic OpenAPI documentation. This implementation uses Fastify as the server with custom React SSR integration. Fastify's encapsulation model keeps plugins isolated, while hooks provide fine-grained request lifecycle control. Ideal for teams with existing Node.js/Express experience who want better performance and structure. Fastify's plugin ecosystem covers everything from rate limiting to WebSocket support.

Leptos

Leptos

VPS (Rust)

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A full-stack Rust framework using fine-grained reactivity. Leptos compiles to WebAssembly for near-native browser performance, with server functions that run seamlessly on the backend. The framework's signal-based reactivity (similar to SolidJS) enables surgical DOM updates without a virtual DOM. Server functions provide RPC-like calls with full type safety between client and server. Leptos can render on the server and hydrate on the client. For Rust developers, Leptos offers a compelling way to build web applications with the safety and performance guarantees Rust is known for. The learning curve is steep but the results are impressive.

Performance Metrics

Lower is better for all metrics. Green indicates the faster framework.

Metric Fastify + React Leptos Winner
TTFB (p50) 42.95ms 44.85ms ◀
TTFB (p95) 46.06ms 50.07ms ◀
LCP (p50) 70ms 72ms ◀
LCP (p95) 78.2ms 78.2ms =
CLS 0 0 =
TBT 0 ms 0 ms =
LCP → TTFB N/A N/A -
LCP → Load Delay N/A N/A -
LCP → Load Duration N/A N/A -
LCP → Render Delay N/A N/A -
Performance Score 99 98 ◀
SEO Score 92 83 ◀
Accessibility Score 80 86 ▶
Best Practices Score 100 100 =

LLM Understanding (Claude Opus 4.5)

How efficiently Claude Opus 4.5 understands the codebase. Based on exploration + validation tests.

Metric Fastify + React Leptos Winner
Exploration Time 3580ms 3206ms ▶
Exploration Tokens 3244 2884 ▶
Files Examined 10 5 ▶
Validation Accuracy 80% 72% ◀
Time Per File 358ms 641ms ◀
Tokens Per File 3060 4138 ◀
Efficiency Score 27.95 29.54 ▶

LLM Understanding (Claude Haiku 4.5)

How efficiently Claude Haiku 4.5 understands the codebase. Based on exploration + validation tests.

Metric Fastify + React Leptos Winner
Exploration Time 1869ms 2152ms ◀
Exploration Tokens 3278 2880 ▶
Files Examined 10 5 ▶
Validation Accuracy 68% 61% ◀
Time Per File 187ms 430ms ◀
Tokens Per File 3061 4098 ◀
Efficiency Score 26.22 34.01 ▶

Build Metrics

Metric Fastify + React Leptos Winner
Bundle Size (JS) 283.6KB 1050.2KB ◀
Bundle Size (CSS) 23.1KB 0KB ▶
Build Time 0s 0s =
Dependencies 21 15 ▶

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