BsonClassMapSerializer.SerializeClass (https://github.com/mongodb/mongo-csharp-driver/blob/dc2dbb5ff06fdea80b44a8c2295e542f166ccb16/src/MongoDB.Bson/Serialization/Serializers/BsonClassMapSerializer.cs#L579)

calls .ToList() unconditionally like this:

var remainingMemberMaps = _classMap.AllMemberMaps.ToList();

This call can easily be avoided by moving the if check at https://github.com/mongodb/mongo-csharp-driver/blob/dc2dbb5ff06fdea80b44a8c2295e542f166ccb16/src/MongoDB.Bson/Serialization/Serializers/BsonClassMapSerializer.cs#L584 into the loop at https://github.com/mongodb/mongo-csharp-driver/blob/dc2dbb5ff06fdea80b44a8c2295e542f166ccb16/src/MongoDB.Bson/Serialization/Serializers/BsonClassMapSerializer.cs#L602

This helps to reduce allocations which can get substantial as shown by the following profiler output of the serialization of 2m+ complex documents: