Yahoo CEO Marissa Mayer has begun firing as many as 500 under-performing employees, All Things D's Kara Swisher reports.

After joining the company in July 2012, Mayer instituted new employee performance reviews one year ago.

The reviews were part of Mayer's plans to trim the Yahoo workforce "very surgically, very carefully," according to a source close to the company.

Now, Swisher reports, Mayer is planning to let go any employees who were rated "misses" or "occasionally misses" at least twice during the past five quarters.

Swisher says as many as 500 employees could eventually be effected. She says that some Yahoo employees are already being let go.

Yahoo has many thousands more employees than many industry experts believes it needs to have.

Before Mayer took over Yahoo, top executives there had plans to cut headcount from ~15,000 down to ~4,000. Those cuts would have involved spinning out Yahoo's search business.

Right when Mayer took the job, famed venture capitalist Marc Andreessen suggested she should get rid of 10,000 people.