What would cause the Cloudwatch to Opensearch lambda to timeout?

Are you using a VPC-enabled Lambda function to stream logs to the OpenSearch cluster? If so, have you verified that the Lambda function has access to the cluster? Depending on the type of security controls in place (e.g., fine-grained access control), there are different configurations you can check. The following documentation has a flowchart you can follow to troubleshoot the timeout issue.

• The bigger picture: fine-grained access control and OpenSearch Service security

Additionally, I recommend adding verbose logging in your Lambda function code to help you understand where it is timing out if there are multiple logics being performed.

Make sure that your Lambda function is idempotent and you should try initialize your function's static logic outside of the function handler. Using CloudWatch Logs for Lambda to your investigation viewing all logs generated by your function's code and identify potential issues. "https://aws.amazon.com/premiumsupport/knowledge-center/lambda-troubleshoot-invocation-timeouts/"