Since github exists, They popularize the concept of pull request (Prs) when we ship code. In order to get the benefits of code review we use Prs as a way to review what we deliver.
In this process are involve, at least, 2 humans[1]:
A requester (Who ask for the CR), and a reviewer (Who do the code review).
But sometimes a reviewer can have a lot to review, a requester ask for somethign who requiere significant effort to understand, round trips between parts. So it ends with a lot of human time and effort.