Cold Start Zero

Abstract

Serverless promised instant scaling but delivered cold starts—the latency penalty when a new function instance spins up. For real-time validation, API gateways, and user-facing interactions, Python's 200-500ms cold start is a UX killer. Rust eliminates this tax with 10-50ms cold starts, 10x faster execution, and 10x lower costs. This paper examines when and how to make the switch.

1. The Hidden Tax on Every Request

Serverless promised instant scaling. What it delivered was cold starts—the latency penalty when a new function instance spins up. For most applications, this is acceptable. For real-time validation, API gateways, and user-facing interactions, it's a UX killer.

Language	Cold Start	Warm Start	The Gap
Python	200-500ms	20-100ms	180-400ms penalty
Node.js	150-400ms	10-50ms	140-350ms penalty
.NET (pre-AOT)	800-3000ms	50-200ms	750-2800ms penalty
Rust	10-50ms	1-5ms	9-45ms penalty

When your validation runs on every keystroke, that 400ms Python cold start isn't an edge case—it's the experience for the first user of every scaled instance.

2. Why Python is Slow to Start

Python's cold start includes multiple initialization steps that compound:

1. Runtime initialization — The Python interpreter must load
2. Package imports — Every import executes code
3. Dependency trees — NumPy, Pandas, etc. have deep initialization
4. Garbage collector setup — Memory management overhead

Even "optimized" Python with stripped dependencies hits 150-200ms minimum.

3. Why Rust Starts Instantly

Rust compiles to a native binary. There's no runtime to initialize:

1. No interpreter — Machine code executes directly
2. No imports — Dependencies compiled into binary
3. No GC — Memory managed at compile time
4. Minimal bootstrap — Just load and run

A Rust Lambda is functionally identical to a C program in startup characteristics, with modern language safety.

The Core Insight

Rust doesn't have a "fast cold start." Rust barely has a cold start at all. The 10-50ms you see is primarily AWS container initialization, not language overhead.

4. Real-World Impact

Scenario: Validation on Form Submission

User fills out a form. Submits. Validation runs.

Language	Cold Start Chance (5% traffic)	P95 Latency
Python	1 in 20 users	450ms
Rust	1 in 20 users	55ms

That 1-in-20 user with Python gets a half-second delay. With Rust, they don't notice.

Scenario: API Gateway at Scale

1000 requests/second, auto-scaling from 10 to 50 instances during spike:

Language	New Instances	Cold Start Delay	User Impact
Python	40	400ms each	4% of requests delayed
Rust	40	40ms each	Imperceptible

5. The Cost Multiplier

Lambda bills by execution time (rounded to 1ms). Faster execution = lower cost.

Language	Avg Execution	Monthly Cost (1M invocations)
Python	150ms	$2.50
Rust	15ms	$0.25

Faster execution means lower costs. The actual savings depend on your workload profile—CPU-bound tasks see the biggest gains, while I/O-bound tasks may see less dramatic improvement since network latency dominates.

6. When Python Still Wins

Rust isn't always the answer:

• Data science workloads — NumPy/Pandas ecosystem unmatched
• Rapid prototyping — Python is faster to write
• ML model inference — Python bindings to TensorFlow/PyTorch
• Batch processing — Cold start irrelevant for long-running jobs

When Rust Wins

7. The Migration Path

You don't rewrite everything. You identify the hot path:

1. Profile your Lambda fleet — Find functions with high invocation + latency sensitivity
2. Extract the core logic — The CPU-bound validation, not the I/O
3. Rewrite in Rust — Using cargo-lambda for AWS integration
4. Keep Python for orchestration — If needed, Python calls Rust microservices

Production Considerations

Rust Advantages	Rust Challenges
Memory safety without GC	Steeper learning curve
Fearless concurrency	Longer compile times
Predictable performance (no GC pauses)	Smaller ecosystem for some domains
Small binary size	Fewer developers available

8. Conclusion

For serverless functions where latency matters, Rust eliminates the cold start tax that Python users have accepted as inevitable.

It's not about Rust being "better." It's about Rust being better for this use case—real-time, user-facing, latency-sensitive workloads where every millisecond of cold start is a UX regression.

References

• AWS Lambda Execution Environment — AWS documentation on Lambda lifecycle
• Rust Runtime for AWS Lambda — AWS blog on Rust Lambda performance

Want to know more about Rust for serverless? Contact me, I'm always happy to chat!