That statement is technically false, but it contains a grain of practical truth that often confuses people.

Here is the breakdown of why that statement is misleading and where the misunderstanding comes from.

The Verdict: False

Rust is almost invariably faster than Python, even in IO-bound tasks.

If you have a web server handling 10,000 concurrent connections that are all waiting on a database (pure IO-bound), the Rust server will use significantly less RAM and CPU to manage those “waiting” connections than the Python server.

Why the statement is wrong (The Technical Reality)

The argument assumes that “IO-bound” means the CPU does zero work. That isn’t true. Even in an IO-heavy application, the application server must do the following:

1. Event Loop Management
   The server has to track which connections are waiting and which are ready to resume.

   • Rust’s async runtimes (like Tokio) are incredibly optimized and have near-zero runtime overhead.
   • Python’s asyncio loop has significant overhead because it is still running interpreted Python code to manage the tasks.

2. Serialization/Deserialization
   When the database replies, the server receives raw bytes. It must turn those bytes into objects (JSON parsing, ORM model instantiation). This is CPU-bound work.

   • Rust does this near-instantly.
   • Python is notoriously slow at object instantiation.

3. The GIL (Global Interpreter Lock)
   Even if your code is async, Python can only execute one bytecode instruction at a time per process.

   • If 100 requests come back from the DB at the exact same millisecond, Python has to process them one by one.
   • Rust can process them in parallel across all CPU cores.

Where the statement is “Practically” True (The Grain of Truth)

The person making that statement is likely conflating “faster” with “indistinguishable to a human.”

If a database query takes 100ms:

• Rust overhead: 0.1ms → total response ≈ 100.1ms
• Python overhead: 5.0ms → total response ≈ 105.0ms

To the human user, 100.1ms and 105ms feel exactly the same.

In this specific context, you could argue that Python is “just as fast” as Rust because the bottleneck is the database, not the language. But it is incorrect to say Python is faster.

The “Fediverse” Exception (Why it matters for PieFed)

The statement “web servers are IO-bound” is often true for something like a simple blog.
It is less true for the Fediverse.

ActivityPub (the protocol PieFed and Lemmy use) involves two things that are heavily CPU-bound, not IO-bound:

1. JSON Parsing
   Fediverse servers throw massive JSON blobs at each other constantly.

   • Python is slow at parsing JSON compared to Rust.

2. Cryptography (RSA Signatures)
   Every time a server sends a message to another server, it must cryptographically sign it (HTTP Signatures). Every time it receives a message, it must verify the signature.

   • Rust handles crypto operations natively and extremely fast.
   • Python relies on C-extensions (like cryptography), which are fast, but the overhead of calling back and forth between Python and C for every single request adds up.

Conclusion

The statement is false.

• Rust is faster at raw execution.
• Rust is faster at handling high concurrency (even IO-bound).
• Python is only “faster” in one metric: development velocity – you can write the code faster in Python, but the code itself will not run faster than Rust.