Enable response caching in Amazon API Gateway is the correct choice because it directly reduces latency without changing the backend. The cache stores method responses in memory keyed by request parameters so calls can be served from the cache instead of invoking Lambda.

The startup regenerates the S3 file every 18 hours so a suitably configured TTL will serve most traffic from the cache and greatly reduce Lambda invocations and cold starts. API Gateway caching is configured at the stage level so you can enable and tune it without modifying Lambda code or S3 output.

Configure Amazon CloudFront to cache API responses ahead of API Gateway can help for global edge caching and reduce latency for distant clients. It is more complex to set up because you must manage cache key and header policies and you may need to adjust origins and behaviors. For a quick drop in fix with no backend changes the built in API Gateway cache is simpler.

Turn on CORS on the API Gateway stage only affects browser cross origin access and does not reduce backend execution or latency. Enabling CORS will not resolve the higher latency experienced after the traffic surge.

Add Amazon ElastiCache to maintain a memory cache for hot keys would improve performance but it requires code changes to read and populate the cache and it adds infrastructure to manage. The team wanted to avoid altering the backend so ElastiCache is not a suitable drop in solution.

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