Performance: successful commit path re-deserializes the just-inserted BSON document

[AGiorgetti]

Summary

After a successful insert, Commit() returns commitDoc.ToCommit(_serializer). This means the write path serializes the commit to BSON for insert, then immediately deserializes the same BSON back to ICommit to return it — pure redundant work.

Why this matters

• Post-insert CPU and allocation overhead on every successful commit.
• The cost is independent of MongoDB round-trip latency.
• Likely noise relative to total write-path time, but worth quantifying before dismissing.

Proposed investigation

• Measure the post-insert deserialization cost as a fraction of total write-path time using MemoryDiagnoser.
• Evaluate building the return ICommit from the original in-memory object instead of round-tripping through BSON.
• Only act if the cost is measurable (> ~1% of total write time).

Reference

See docs/Performance-Investigation.md → Finding #5 for full context.