Since GitHub became popular, so did the concept of pull request (PR) for shipping code. In order to get the benefits of code review we use PRs as a way to review what we deliver.
In this process at least 2 humans are involved[1]:
A requester (who asks for the code review), and a reviewer (who does the code review).
But sometimes a reviewer can have a lot to review, or a requester can ask for something that requieres significant effort to understand and round trips between the two parts. So it ends up costing a lot of human time and effort.