Using the user ID as the primary key in a large table


I have a table (id (PK, UNIQUE), user_id, [other columns here]) with a large amount (100k+) of users' orders, where each user can access only orders that have set user_id of him.

I wonder how using user_id as primary key could affect performance of SELECT queries on this database. Is there any efficient solution to maintain large amount of rows binded to users (each order have one owner) in a single InnoDB table?

If most queries include a specific value for user_id, this is more efficient:

PRIMARY KEY(user_id, id),  -- to get the benefit of 'clustering' around user_id
INDEX(id)  -- to make AUTO_INCREMENT happy