Why does using $projection higher up in a pipeline not improve performance?

Using $projection higher up in a pipeline does not necessarily improve performance because MongoDB is already optimized to determine which fields are needed for the operations that follow. When a query is executed, the MongoDB engine analyzes the query and evaluates the relevant fields to retrieve, only reading the data that is necessary for that specific query operation.

In this context, the database is inherently designed to minimize data transfer and unnecessary computations. As a result, moving the $projection stage earlier in the pipeline may not yield performance benefits because MongoDB is already efficient in its data handling. It focuses on delivering the required fields based on query requirements and indexing, rather than reprocessing or recalculating which fields are necessary.

By acknowledging that MongoDB calculates what fields it needs based on its internal mechanics, we can understand that there might be no significant gain or reduction in performance from adjusting the position of the $projection stage. This highlights the importance of understanding how MongoDB optimizes queries and handles data retrieval internally.