This is not true. It is not nearly immediate at US consulate and backlogged in US. The parent doesn't know what they are talking about.

Because America only has a few processing centers in within the US where is that literally hundreds and hundreds of consulates that can now take on this activity they have always been doing this activity but the vast majority of the backlog is caused by the slow processing of the US processing centers.

USCIS serializes it and they have a limited number of workers. CP shards it based on country so it will be much faster for many people.