Performance issue with composite primary key or not on Transactional Replication with AlwaysOn Subscribers?
Performance not only depends on the query, but also the kind of data you store in the table.

Let's zoom out and talk about how you choose a clustering key for a table. Generally, you want the clustering keys to be:


Static - non-changing
Unique
Narrow - as few fields as possible, with as small datatypes as practical
Ever-increasing - which gives you a hotspot at the end of the table, which would sound bad but is usually good in small (sub-terabyte) database designs


Here, you've already got a single field that satisfies all of those requests: CLIENT_ID, which you defined as an identity field. You could just start by using that field alone - adding SERVER_ID doesn't really buy you anything.

However, I'm a little suspicious of the table design because SERVER_ID is only a CHAR(1). That gives you a limited number of servers to work with per CLIENT_ID. I don't know what you're trying to model, but generally an ID wouldn't be a single character.