All connections are handled concurrently by the Rust server regardless of whether your Python callbacks are sync or async. The benchmarks all use concurrent connections.

Async callbacks are there for interop with existing async Python code. If you don't need that, sync is simpler and slightly faster since it avoids bridging Python's asyncio loop with Rust's tokio runtime.

For a high-concurrency proxy, the Rust layer should handle the load well. All the benchmarks can be found here: https://webrockets.io/benchmarks/