I used DynamoDB as part of the job a few years ago and never got single-millisecond responses - it was 20ms minimum and 70+ on a cold-start, but I can accept that optimising Dynamo's various indexes is a largely opaque process. We had to add on hacks like setting the request timeout to 5ms and keeping the cluster warm by submitting a no-op query every 500ms to keep it even remotely stable. We couldn't even use DAX because the Ruby client didn't support it. At the start we only had a couple of thousand rows in the table so it would have legit been faster to scan the entire table and do the rest in memory. Postgres did it in 5ms.

If Amazon said they didn't use DAX that day I would say they were lying.

The average consumer or startup is not going to squeeze out the performance of Dynamo that AWS is claiming that they have achieved.

In fact, it might have been fairer in Ruby if they didn't hard-code the net client (Net/HTTP). I imagine performance could have been boosted by injecting an alternative.