Couldn't this be a problem for application authors and a potential source of DoS activities? It seems this should be a big red warning flag somewhere.
All databases tend to struggle to under contention. If you are an application author building a high-scale app, you need to avoid contention by design. That single counter row you were planning to increment on every visit? Bad idea; maybe insert a new visit record instead.
The contention in this example is an aspect of the test design, not database design. You wouldn't try to build an app this way.