Explain the differences between using `populate()` and performing manual joins in Mongoose when retrieving related data from multiple collections, with a focus on performance.

Both `populate()` in Mongoose and manual joins aim to retrieve related data from multiple collections, but they differ significantly in their implementation and performance implications. `populate()` is a Mongoose-specific feature that automates the process of replacing specified paths in a document with actual documents from another collection. It relies on defined `ref` options in Mongoose schemas, which establish a relationship between two collections (e.g., a `User` document referencing `Post` documents). When `populate()` is called, Mongoose performs a separate query to retrieve the related documents and then merges them into the original document. Manual joins, on the other hand, involve writing custom aggregation pipelines or performing multiple queries and manually combining the resu....