WebAssembly - When JavaScript Isn't Fast Enough

WebAssembly is basically a way to run compiled code in browsers. Built because JavaScript is slow as hell for anything computationally intensive, WASM lets you compile C, C++, Rust, and other languages to run in browsers at reasonable speed.

But let's be real about the performance claims floating around. The marketing says "near-native speed" - that's bullshit. USENIX research from 2019 found WASM runs 45-55% slower than native code on average, with some workloads hitting 2.5x slower. Recent benchmarks show similar results: Go WASM is 13x slower, Python WASM is 23% slower, and Rust WASM is about 5x slower than their native versions.

Why Companies Actually Use It

Companies use WASM when they need performance and JavaScript makes them cry. Figma uses it for graphics rendering because canvas operations in JavaScript are painfully slow - their implementation cut load times by 3x. Adobe uses it in Premiere Rush because video processing in JS would be unusable. Cloudflare uses it in Workers because edge functions need to start fast and run efficiently, handling 10M+ requests per second.

The real value isn't speed - it's code reuse. If you have a C++ image processing library that works, you can compile it to WASM instead of rewriting everything in JavaScript. The performance hit is painful but better than starting from scratch.

The Reality of WebAssembly Development

Don't use WASM unless:

1. You have CPU-intensive code that's actually slow in JavaScript
2. You have existing C/C++/Rust code to port
3. You enjoy debugging problems with terrible tooling

For 90% of web apps, just optimize your JavaScript first. WASM comes with serious baggage: build complexity increases 10x, debugging tools are primitive, and your bundle size will explode with the runtime and glue code. WASM modules often include entire standard libraries, leading to bloated file sizes even for simple functions.

If you're still determined to use WASM after reading all that, start with the official Mozilla WebAssembly documentation or the WebAssembly.org developer guide. Both will give you a more optimistic view of the technology than you'll find here.

So you've decided to ignore the warnings and try WASM anyway? Welcome to the real world of WebAssembly development.

Getting Your C++ Code to Actually Compile to WASM

Here's what nobody tells you about the WASM build process: it's a fucking nightmare. You'll spend more time fighting Emscripten than writing code.

The Emscripten Reality:

## This looks simple, right? WRONG.
emcc main.cpp -o main.wasm

## What actually happens:
## - Emscripten breaks on new libc versions
## - Your build randomly fails on different machines  
## - Memory allocation bugs that don't exist in native builds
## - Mysterious linker errors with no helpful messages

I've debugged builds where changing the order of includes fixed compilation. I've seen projects that compile fine on Ubuntu 20.04 but fail on 22.04 because of subtle libc differences that Emscripten can't handle. The CMake integration alone is a source of endless frustration.

WASI: Still Half-Baked After All These Years

WASI was supposed to solve the "write once, run anywhere" problem. Instead, we got version fragmentation:

WASI Preview 1 (2020-2024): Basic file I/O that worked sometimes. No networking, no threading, barely usable for real applications.

WASI 0.2 (2024): Introduced the Component Model that nobody understands and wasi-http that works differently in every runtime.

WASI 0.3 (expected mid-2025): Promises async I/O support that will probably be incompatible with 0.2.

The problem isn't the standards - it's that every runtime implements them differently, and the tooling is years behind where it needs to be.

Production War Stories

Cloudflare Workers work because Cloudflare controls the entire stack and has teams of engineers making WASM not suck. Your experience will not be the same.

I've seen teams spend 6 months migrating a C++ image processing library to WASM, only to discover:

• Binary bloated from 2MB to 8MB with the runtime
• Performance was 60% slower than expected
• Debugging crashes required printf statements and reading assembly
• Memory leaks that were impossible to track down

One team took down prod for 2 hours because a WASM module had a memory leak that only manifested under high load. The error message was just "out of memory" with no stack trace. Memory debugging in WASM is particularly painful because there's no automatic garbage collection.

The Component Model: Theoretical Perfection, Practical Hell

The Component Model is supposed to let different WASM modules talk to each other. In practice:

• WIT syntax is like learning another programming language
• Interface versioning is a disaster when types change
• Cross-language calls have unpredictable performance overhead
• Error handling across component boundaries is broken

I tried to get a Rust WASM module talking to a C++ WASM module. Spent a week fighting type serialization before giving up and just using message passing.

The bottom line: WebAssembly works, but you'll earn every bit of performance you get. If these war stories sound familiar, the FAQ section covers the specific problems you're probably about to run into.