Why do large documents negatively impact write performance in MongoDB?

Large documents negatively impact write performance in MongoDB primarily because the entire document needs to be rewritten to storage whenever a change is made. This process is due to the way MongoDB handles document updates, which often involves creating a new version of the document and then replacing the old version.

When a document is large, the operation of copying the entire document into a new location and then updating the index can be time-consuming and resource-intensive. This can lead to increased latency and reduced throughput during write operations, especially when many large documents are being updated frequently.

Additionally, larger documents can lead to increased fragmentation in the storage engine, as documents may not fit perfectly into the allocated storage blocks, potentially causing further decreases in performance due to inefficient use of storage resources.

In contrast, smaller documents are generally more efficient to update since they involve less data to manage during an update. This is why managing document sizes is a critical consideration for optimizing write performance in MongoDB.