Agreed. I think it is highly unlikely that PIP is being used on a large scale as a real cost-cutting measure. PIP takes months. More likely, they simply are realizing that they have some duds on staff. They've grown tremendously in the past four years, and not all those hires were gems.

I have even heard that there are new hiring rules, that engineers now have to have X years of experience doing Y before they can be hired. Which is totally anti-old-school-Google; they would hire the greenest college kid if s/he seemed brilliant and industrious. This suggests that they now believe they have a systemic hiring problem. Or, that brilliance is now not as important as experience. Either way, it suggests that Google is changing.