BSONElement construction was made more expensive, which showed up as costs to various change_streams workloads.

DocumentStorage in particular has patterns of BSONElement construction where multiple BSONElement constructions are performed on the same underlying element. We should look to save the BSONElement or the BSONElement sizes here. 

There are also cases where we might want to cache the results of BSON iteration. getField is performed on the same underlying BSONObj.