If Replication Factor and Consistency Level are set to QUORUM then we can achieve Availability and Consistency but Performance degrade will increase as the number of nodes increases.
Is this statement correct? If yes then what is the best practice to get better result, considering Availability and Consistency as high priority and not to decrease performance as number of nodes increases.
Not necessarily. If you increase the number of nodes in your cluster, but do not alter your replication factor, the number of replicas required for single partition queries does not increase so you should therefore not expect performance to degrade.
With a 10 node cluster, replication factor 3 and CL QUORUM, only 2 replicas are required to meet quorum, the same is true for a 20 node cluster.
Things change if your query requires some kind of fan out that requires touching all replica sets. Since you have more replica sets, your client or the coordinating C* node needs to make more requests to retrieve all of your data which will impact performance.
