Is FastAPI Not Fast Enough? A Direct Performance Comparison with Robyn

To build an API Gateway in front of an AI inference engine, Robyn was chosen instead of FastAPI, and the key reason was throughput and stability under high load conditions.

Robyn is a Python web framework that runs on a Rust runtime, emphasizing an architecture where Rust handles the event loop and workers to reduce the constraints of the Python layer. Even at the level of simple /health, /data example code, it can be used with syntax similar to FastAPI or Flask, but underneath, a Rust-based multithreaded runtime handles request processing and socket handling.

When benchmarked in the same VM environment, both frameworks were similar at around 1.1ms for a single request, but under a 50 concurrent users condition, Robyn recorded 763.28 RPS and FastAPI recorded 651.02 RPS, with Robyn ahead by 17.2%. Under the same conditions, P99 latency was 96.4ms for Robyn versus 286.4ms for FastAPI, and maximum response time was also much lower for Robyn at 143.4ms compared to FastAPI's 674.8ms.

The difference became clearer under greater load.

• In the 1,000 concurrent users test, FastAPI was faster in average response time at 0.1141s, but Robyn was far more stable at P95/P99.
• Robyn: P95 0.2775s, P99 0.3432s
• FastAPI: P95 0.9216s, P99 1.7410s

In other words, even though some FastAPI requests were fast, it collapsed sharply in the upper latency range, while Robyn maintained a tighter response distribution. The article interprets this as a difference in tail latency resilience.

In the most extreme -c 12000 test, Robyn successfully processed all 12,000/12,000 requests, while FastAPI only succeeded on 7,809/12,000, with 4,191 EOF / Reset errors occurring. Beyond simple average speed comparisons, the conclusion is that Robyn is better suited in terms of survivability when withstanding large-scale burst traffic.

In conclusion, Robyn's strengths are cited as performance, stability, and retaining Python syntax, though as a new framework, its ecosystem maturity and references should still be viewed conservatively. Nevertheless, the article assesses that Robyn is a practical choice in environments where high load and tail latency matter, such as a Gateway for AI/ML serving.