Why is the origin region Redis not also a reader of the replication stream? Just for latency?

yep. in this specific use case there can be pretty fast reads after a write, and dynamo stream latency is not great even intra-region.

Via the lambda integration you should probably expect ~1000ms p99 latency intra-region, and ~2100ms p99 from, e.g, east-1 to west-2. Shave maybe 200-300ms off of those numbers if you do a DIY stream consumer without the aws provided lambda connector.