Boat owners will know that you have to scrape the barnacles off the hull of a boat at regular intervals to improve performance. The same is true of a business, and the barnacles are under-performing employees.
Every year, the CEO should look at the employee base, identify who are the least productive and terminate them. In larger companies it is the bottom 10%. In smaller companies, it may be as few as one employee.
The most common objection I hear to this is that it will unsettle the employees because they know that their jobs are not automatically safe. In reality the only ones unsettled are the bottom 10% - and you are on your way to producing a high-performing culture.