Anecdotally, my experience running Redis servers on AWS (both standalone EC2 instances and ElastiCache dedicated Redis instances) is that network latency is likely to become a barrier before anything else. We struggled with the same performance problems on both small-footprint small-payload Redis DBs and on large ones, and paying for the next tier of network connectivity (between our applications on EC2 and our Redis servers on EC2/ElastiCache) did the most to alleviate delays.