This issue is critical to improve $merge performance in mongostream. If the fix for this problem is also helpful to improve the performance of applying updates to unique docs (i.e. {multi: false}), that would be wonderful as well.
Here is a pr to reproduce this performance gap: https://github.com/10gen/mongo/pull/22857
Please look atĀ HELP-60116 for more context on why ASP team cares about this.