It just depends on the severity of the problem. Some problems can be fixed (PIP), while others don’t warrant the attempt - just use your imagination. If employer is an at-will, they certainly have more flexibility to fire quickly. They may increase the odds of a wrongful termination suit by doing this, but that’s what insurance is for. Nobody wants to pay a wrongful term suit though, and usually a company dousing an employee will have enough of a reasonable cause they can piece together after the fact. Shit gets dicey if employee age is above the mean or is on FMLA, but generally an at-will employer has a lot of flexibility to drop an employee.
My experience with PIPs is that they're essentially an advance notice that you're getting fired and a grace period while you finish up outstanding projects and look for another job.
There's a lot of flexibility with at will employers, but even for clear firings with just cause, it's often less risky to eat the cost of that grace period. Aside from the risk of (frivolous) legal costs, there are reputational and morale risks from departures that aren't adequately "managed." No one is ever walked into an office and fired on a Friday morning for performance reasons without lots of warnings, including a PIP.
