so this is not a
no answer, but let me try to elaborate. In your above case I guess it will not speed up things significantly, as all of your nodes are probably already answering search queries - thus it is a question of existing load plus the ability in this case, that all of the data is locally available, so you could prevent network roundtrips. You could also probably just use an index with 1 shard and two replicas, so that all nodes hold the data in a single shard, speeding up queries - if that is possible.
So, what is meant with the explanation of adding more shards to have faster reads? Imagine having one node with one shard. This node is able to sustain 5000 queries per second. Now your task is to scale the system to 25000 queries per second. Then adding five more nodes to the cluster and configuring the number of replicas to 5 would mean that each node had a copy of that shard, totaling 6 shards. In this setup your cluster would be able to sustain 30k queries per second (a little headroom might be a good idea).
Hope this explanation makes a bit more sense. Using replicas to scale reads means the ability to be able to answer more search queries in parallel.
Hope this helps!