This keeps getting repeated on this thread but its not true. U.S. WARN laws require severance or 60 days notice for mass layoffs (which would definitely include 5% of a company as large as GM). Only terminations for cause are exempted, and performance is only grounds for cause in a very small handful of "right to work" states.
Only for-cause terminations are excluded the WARN Act, and for-cause in this context means specifically behavior that violates the law, or the employee handbook setting forth company rules (which are binding on both the company and employee, which is why companies make you read them every year). Also note that companies can't make poor performance a violation of company rules; this has been tried before and smacked down pretty harshly even in right-to-work states.
Thus, those 5% laid off due to stack ranking are entitled to 60 days of severance, or 60 days notice of an impending termination.
Think of this is as a continuous process of evaluation -> pip -> fire. This means no WARN and no severance.
And the part where rolling layoffs at a company the size of GM will all involve multiple layoffs exceeding the WARN threshold.
And the part where no general counsel will let their company knowingly subject itself to a $500 daily fine per employee, in addition to full back pay for the mandated period.
And the part where GM satisfies its WARN obligations by paying the mandated severance (or more) in lieu of providing advance notice...which is what most companies do...