So there's a couple big questions:
1. how big and how structured is the company? A 100-500 person everything is still ultimately decided by the founder is very different than a 20,000 person publicly traded firm.
2. Is this a "poor" performance review or a PIP?
3. How's corporate culture? Is there a cabal involved?
3 What's the nut of the issue?
4. Is it free of any potential related discriminatory issues? Race, sex, age, orientation, etc?
Having fired, 'helped people find the door' and having needed to find the door, my viewpoint is that in larger firms, HR's primary role is protecting the company. They'll make sure the established process is followed and that the appearance of potential discriminatory behavior isn't involved.
In a smaller firm that's still founder driven, they will do whatever the founder wants. Smart founders have them operate like the above. Not smart founders have the appearance of being capricious.
If you've crossed the Cabal, even a bigger firm will operate more like a capricious founder. Although you're more likely to pick up a severance package.
JIMHO, YMMV