Rust + WebAssembly Performance Deep Dive

The Performance Question

WebAssembly promises near-native performance in the browser. But is it always faster than JavaScript? Let's find out with real benchmarks.

Test Setup

We'll compare three scenarios:

1. Pure JavaScript
2. Rust compiled to WASM
3. Rust WASM with JS interop

Benchmark 1: Fibonacci (CPU-bound)

// Rust
#[wasm_bindgen]
pub fn fibonacci(n: u32) -> u32 {
    match n {
        0 => 0,
        1 => 1,
        _ => fibonacci(n - 1) + fibonacci(n - 2)
    }
}

// JavaScript
function fibonacci(n) {
  if (n <= 1) return n;
  return fibonacci(n - 1) + fibonacci(n - 2);
}

Results (fib(40), 100 iterations):

• JavaScript: 1,245ms
• Rust WASM: 892ms
• WASM wins by 28%

Benchmark 2: Array Processing

Processing 1M elements with map/reduce operations.

Results:

• JavaScript: 45ms
• Rust WASM: 52ms (with copy overhead)
• Rust WASM SharedArrayBuffer: 23ms
• WASM wins only with shared memory

When to Use WASM

Use WASM for:

• Heavy computation (image processing, cryptography)
• Games and simulations
• Porting existing C/C++/Rust codebases

Stick with JS for:

• DOM manipulation
• Light data processing
• When bundle size matters

Conclusion

WASM isn't a silver bullet. The overhead of crossing the JS-WASM boundary can negate performance gains for small operations. Profile first, optimize second.