Why does my Lambda function timeout even when it runs fast locally?

There are several reasons why your Lambda function might time out even when it runs quickly on your local machine:

1. Initialization overhead: Lambda functions have an "Init" phase that runs before your code executes. If this phase takes too long, it can consume a significant portion of your timeout duration. This is especially true for "cold starts" when a new execution environment is created.

2. Resource constraints: Lambda functions might have less CPU power than your local machine. The CPU allocation is proportional to the memory you assign to your function. If you've configured your function with minimal memory, it will have less processing power.

3. External dependencies: Your function might be interacting with other services (like S3, databases, or external APIs) that respond more slowly in the cloud environment than in your local tests.

4. Data size differences: Your local tests might use smaller data samples than what the function processes in production. Larger downloads from S3 or more complex data processing can significantly increase execution time.

5. Network latency: If your function makes network calls, the latency in AWS might be different from your local environment.

To resolve these timeout issues, you can:

• Increase the function's memory allocation, which also increases CPU power
• Optimize your initialization code to reduce startup time
• Increase the timeout setting beyond 10 seconds if needed (up to 15 minutes)
• Ensure your tests use realistic data sizes and parameter values that match production scenarios
• Consider asynchronous processing for long-running tasks

Remember that Lambda execution times can vary based on factors like data transfer size, processing complexity, and the responsiveness of other services your function interacts with.
Sources
Troubleshoot invocation issues in Lambda - AWS Lambda
Configure Lambda function timeout - AWS Lambda
Troubleshoot execution issues in Lambda - AWS Lambda
Lambda API returns 200 on request timeout error | AWS re:Post