Cold Starts Are Dead

It never fails. Every time I talk about serverless, someone pushes back with the cold start argument. I still see it in forums, in blog comments, in architecture review meetings. “Sure, but what about cold starts?”

I get it. Five or six years ago, that was a legitimate concern.

But it’s 2026. The data tells a different story. And if you’re still making decisions based on the cold start argument, you’re arguing against a version of Lambda that hasn’t existed in years.

How Long Are Lambda Cold Starts in 2026?

Let’s start with what cold starts actually look like today. These numbers come from production workloads observed in the wild, not synthetic hello-world tests. Your mileage will vary by package size, initialization code, and memory configuration, but the ranges are representative of what teams are seeing in 2026.

Runtime	P50	P99	Notes
Python 3.13	200-400ms	800ms-1.2s	Fastest scripting runtime
Node.js 22	200-350ms	600ms-1s	Solid general choice
Go	50-100ms	150-250ms	Near-zero
Rust	50-80ms	100-200ms	Fastest overall
Java 21 (no SnapStart)	2-5s	6-10s	Still slow without SnapStart
Java 21 + SnapStart	90-140ms	200-400ms	Dramatically better

Running on arm64 (Graviton)? Knock another 15-40% off those numbers across the board.

Rust cold starts have been measured as low as 16ms on arm64. Sixteen milliseconds, which is less a cold start problem and more a rounding error.

The scripting runtimes, Python and Node, land in the 200-400ms range at P50. For context, that’s less than the time it takes your browser to render a page after receiving the HTML.

Does VPC Still Cause Lambda Cold Starts?

This one still comes up in conversations, and it frustrates me because it was solved in 2019.

The old problem: when a Lambda function needed VPC connectivity, AWS had to create an Elastic Network Interface (ENI) on the fly. That meant 10-15 seconds of additional cold start latency on top of everything else. VPC-connected Lambda was genuinely painful.

AWS fixed this when Lambda migrated to Firecracker microVMs in 2019, dropping cold start overhead from over ten seconds to under a second. Werner Vogels recently wrote about the invisible engineering behind Lambda’s network. The team used eBPF to rewrite Geneve tunnel headers, taking tunnel latency from 150 milliseconds to 200 microseconds. The VPC cold start penalty now approaches zero for most workloads.

That was seven years ago. If someone tells you Lambda cold starts are bad because of VPC, they’re working from outdated information.

How Much Does SnapStart Reduce Cold Starts?

Java was the poster child for the cold start argument. And honestly, it earned that reputation. A Spring Boot app on Lambda could take 3-10 seconds to cold start. That’s painful no matter how you frame it.

SnapStart changed the math. Here’s the timeline:

November 2022: SnapStart GA for Java at re:Invent
November 2024: Python 3.12+ and .NET 8+ GA
2025: Expanded to additional regions, arm64 support

How it works: Lambda takes a snapshot of the initialized Firecracker microVM after your INIT code runs. That snapshot gets cached across three tiers: L1 on the worker, L2 in the placement group, S3 at the region level. On cold start, Lambda restores from the snapshot instead of re-running your initialization code.

The benchmarks are real:

Python (Flask, LangChain, Pandas): several seconds → sub-second
Java Spring Boot: 5.8s → 180ms (97% reduction)
.NET: 58-94% cold start reduction

SnapStart has continued to evolve. Python and .NET support went GA in late 2024, with additional regions and arm64 support following in 2025.

Did the Lambda INIT Billing Change Increase Costs?

In August 2025, AWS standardized billing for the Lambda INIT phase. Functions packaged as ZIP files with managed runtimes now get billed for the INIT phase, which was previously free. This triggered some alarming blog posts.

The headline claim: a “22x cost increase.” Let’s look at the math.

That 22x number requires a perfect storm of worst-case assumptions:

100% cold start rate (every invocation is a cold start)
2-second Java INIT duration
512MB memory configuration

Here’s the problem with that scenario: AWS’s own production analysis shows cold starts occur in less than 1% of invocations. Not 100%. Less than 1%.

AWS’s own assessment: “most users will see minimal impact on their overall Lambda bill from this change, as the INIT phase typically occurs for a very small fraction of function invocations.” The actual impact depends on your cold start ratio and INIT duration relative to handler duration. Use the CloudWatch query below to check your own numbers.

Don’t take my word for it. AWS published a CloudWatch Logs Insights query so you can calculate your exact impact:

filter @type = "REPORT"
| stats
    sum((@memorySize/1000000/1024) * (@billedDuration/1000)) as BilledGBs,
    sum((@memorySize/1000000/1024) * ((@duration + @initDuration - @billedDuration)/1000)) as UnbilledInitGBs,
    UnbilledInitGBs / (UnbilledInitGBs + BilledGBs) as UnbilledInitRatio

Run that against your own functions. If your UnbilledInitRatio is anywhere near the number that would produce a 22x increase, you have a bigger problem than billing changes. You have an architecture problem.

With SnapStart enabled, INIT durations drop to sub-second, which shrinks that ratio even further.

I Decided to See for Myself

I wasn’t satisfied citing someone else’s numbers for a post called “Cold Starts Are Dead.” So I built a benchmarker. Thirteen Lambda functions across six runtimes, both arm64 and x86_64, 50 cold start invocations each, 512MB memory, us-west-2. Minimal hello-world handlers, no frameworks, no dependencies beyond the runtime SDK. This measures the platform floor, not application init time.

Runtime	Arch	P50 (ms)	P99 (ms)	Blog Claim	Verdict
Rust	arm64	14.1	31.9	50-80ms	⚡ Faster
Rust	x86_64	17.0	29.3	50-80ms	⚡ Faster
Go	arm64	45.0	61.2	50-100ms	⚡ Faster
Go	x86_64	59.8	94.9	50-100ms	✅ Verified
Python 3.13	arm64	88.3	147.1	200-400ms	⚡ Faster
Python 3.13	x86_64	106.2	142.3	200-400ms	⚡ Faster
Node.js 22	arm64	121.5	168.5	200-350ms	⚡ Faster
Node.js 22	x86_64	155.0	231.3	200-350ms	⚡ Faster
Java 21	arm64	365.3	539.2	2-5s	⚡ Faster
Java 21	x86_64	443.8	573.5	2-5s	⚡ Faster

Every runtime came in at or below the production ranges I cited earlier. That’s expected. Those production numbers include application dependencies, framework initialization, and SDK client setup. My minimal handlers isolate just the platform overhead. Think of these as the floor: your cold starts will be at least this fast, plus whatever your initialization code adds.

The arm64 advantage verified cleanly too:

Runtime	arm64 P50	x86_64 P50	Improvement
Rust	14.1ms	17.0ms	17% faster
Go	45.0ms	59.8ms	25% faster
Python	88.3ms	106.2ms	17% faster
Node.js	121.5ms	155.0ms	22% faster
Java	365.3ms	443.8ms	18% faster

17-25% faster across every runtime. If you’re still deploying on x86_64 in 2026, you’re leaving performance on the table for no reason.

I also tested VPC vs non-VPC with the Python function. The VPC-connected function was 1.4ms faster at P50, within noise.

One honest caveat: SnapStart on my minimal Java handler showed a ~670ms restore duration. That’s because there’s almost nothing to snapshot. The restore mechanism’s own overhead dominates. For a Spring Boot app where SnapStart eliminates 3-5 seconds of framework init, you’d see the dramatic improvement the benchmarks above describe. SnapStart’s value scales with how much init work your app does.

The full benchmarker is open source on GitHub. You can run it yourself:

git clone https://github.com/singledigit/lambda-benchmark.git
cd lambda-benchmark
sam build && sam deploy --guided
cd orchestrator
python benchmark.py --stack-name <stack> --iterations 50
python generate_report.py

When Do Lambda Cold Starts Still Matter?

Here’s where cold starts still matter.

But first, is 200ms even slow? For a website, a chat app, an API powering a mobile experience, an AI agent? You won’t notice it. A typical page load involves DNS resolution, TLS handshake, and content rendering that add up to far more than 200ms. If you’re building AI agents, the LLM call alone takes 2-10 seconds, so 200ms of cold start is a rounding error on that. It’s the first request after a quiet period, and it happens in less time than a blink.

The cases where cold starts actually matter are narrow:

Sub-10ms latency SLAs. If you’re building for high-frequency trading or real-time bidding, 200ms of cold start latency is unacceptable. At that point, you’re probably looking at containers on ECS or EKS where you control the warm pool directly. Provisioned Concurrency is an option too, but it has its own cost tradeoffs.

Very spiky, unpredictable traffic with strict P99 requirements. If your traffic pattern goes from zero to thousands of concurrent invocations with no ramp-up, and you have hard P99 latency SLAs, cold starts will hit that tail. You have options: containers with pre-scaled task counts, Provisioned Concurrency, or accepting slightly higher P99s during the initial burst. The right answer depends on your cost tolerance and how strict “strict” actually is.

Java without SnapStart. Just… use SnapStart. If you’re running Java on Lambda without SnapStart enabled, you’re choosing to have a cold start problem. The fix is a configuration change.

The honest framing: the question isn’t “do cold starts exist?” They do. The question is “do they matter for your workload?” For the vast majority of workloads, the answer is no.

Conclusion

VPC cold starts were fixed in 2019, and SnapStart took Java from 5 seconds to 180 milliseconds. Python and Node sit under 200ms on arm64, and AWS’s own data says cold starts happen in less than 1% of invocations. I built a benchmarker and tested it myself. The production claims are conservative, and the numbers are better than advertised.

The cold start argument had its day. That day was 2018. If you’re still leading with it, I get it, it used to be real. But the data has moved on, and it’s time we did too.

For the Agents

If you’re using an AI coding assistant or agent, I’ve put together a companion skill file for this post. It includes the 2026 cold start benchmarks, SnapStart configurations, and the CloudWatch query for INIT billing analysis. Everything your agent needs to give you accurate cold start guidance instead of outdated advice.