> At 10k RPS

You would've used local memory first. At which point I cannot see getting to those request levels anymore

> ElastiCache Serverless (Redis/Memcached): Typical latency is 300–500 microseconds (sub-millisecond response)

Sure

> DynamoDB On-Demand: Typical latency is single-digit milliseconds (usually between 1–10 milliseconds for standard requests)

I know very little use cases where that difference is meaningful. Unless you have to do this many times sequentially in which case optimizing that would be much more interesting than a single read being .5 ms versus the typical 3 to 4 for dynamo (that last number is based on experience)