Leptos vs Fastify + React
Real-world performance comparison based on identical SaaS dashboard implementations.
Leptos
VPS (Rust)
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 | Leptos | Fastify + React | Winner |
|---|---|---|---|
| TTFB (p50) | 44.85ms | 227.7ms | ◀ |
| TTFB (p95) | 50.07ms | 484.46ms | ◀ |
| LCP (p50) | 72ms | 360ms | ◀ |
| LCP (p95) | 78.2ms | 945.6ms | ◀ |
| 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 | 98 | 98 | = |
| SEO Score | 83 | 92 | ▶ |
| Accessibility Score | 86 | 80 | ◀ |
| 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 | Leptos | Fastify + React | Winner |
|---|---|---|---|
| Exploration Time | 3206ms | N/A | - |
| Exploration Tokens | 2884 | N/A | - |
| Files Examined | 5 | N/A | - |
| Validation Accuracy | 72% | N/A | - |
| Time Per File | 641ms | N/A | - |
| Tokens Per File | 4138 | N/A | - |
| Efficiency Score | 29.54 | N/A | - |
LLM Understanding (Claude Haiku 4.5)
How efficiently Claude Haiku 4.5 understands the codebase. Based on exploration + validation tests.
| Metric | Leptos | Fastify + React | Winner |
|---|---|---|---|
| Exploration Time | 2152ms | N/A | - |
| Exploration Tokens | 2880 | N/A | - |
| Files Examined | 5 | N/A | - |
| Validation Accuracy | 61% | N/A | - |
| Time Per File | 430ms | N/A | - |
| Tokens Per File | 4098 | N/A | - |
| Efficiency Score | 34.01 | N/A | - |
Build Metrics
| Metric | Leptos | Fastify + React | Winner |
|---|---|---|---|
| Bundle Size (JS) | 1050.2KB | 285.3KB | ▶ |
| Bundle Size (CSS) | 0KB | 23.1KB | ◀ |
| Build Time | 0s | 2.5s | ◀ |
| Dependencies | 15 | 21 | ◀ |
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