Off-the-shelf CMS platforms are almost never built for high-traffic scenarios. The reason is simply that the developers of these systems don't plan for it. The highest-trafficked websites almost always rely on custom solutions, because they know that a high-availability, scaled solution is difficult with off-the-shelf solutions.
Custom CMS Scalability
Why do so few of the largest websites in the world use off-the-shelf software? Because of scale.