When updating document with hundreds of properties, the time spent in update conflict detection is high.

The FieldRefSet::insert is visible in flame graph.

Attached is a js script that reproduce the problem that create a bottleneck in production.

Just for information, we also do update conflict detection on our application before submitting the update commands to MongoDB. We had the same performance problem using a similar algorithm, this has been fixed using a different approach:
https://github.com/nuxeo/nuxeo/commit/eeceab7024445b88082f7406290478d9f6702c28