I was debugging through the code and noticed this inefficiency. It's probably not the only place.

DBCollection:
private void _checkKeys( DBObject o ) {
for ( String s : o.keySet() ){
validateKey ( s );
Object inner = o.get( s );
if ( inner instanceof DBObject )

{ _checkKeys( (DBObject)inner ); }

else if ( inner instanceof Map )

{ _checkKeys( (Map<String, Object>)inner ); }

}
}
There should be an alternative implementation that takes a Map and iterates the entry set, to avoid expensive lookup for every single key in the structure. Method _checkKeys should then start with an instanceof check and delegate to this method, if possible, which it probably will be for the vast majority of cases.