Redis clustering splits the cluster on two axis sharding, and replicas. Each shard is made up of one primary, and n secondary replicas.
Multiple shards provide better aggregate write performance by splitting the keyspace so each shard is responsible for part of the keyspace. However more shards can decrease availability as if a shard is unavailable that part of the keyspace is unavailable. If your write load is concentrated on a single or small number of keys, this may not help.
Multiple replicas provide better read performance as read queries are spread across all replicas in the shard. Write queries happen on the primary and are then copied to the secondaries. Multiple replicas can also improve availability as if any secondary goes down any other secondary in the shard can be queried, whilst if the primary goes down a secondary can be elected to replace it.