DocsStreamer populates SolrDocument w/unnecessary fields

We observe that solr query time increases significantly with the number of rows requested,  even all we retrieve for each document is just fl=id,score.  Debugged a bit and see that most of the increased time was spent in BinaryResponseWriter,  converting lucene document into SolrDocument.  Inside convertLuceneDocToSolrDoc():   

https://github.com/apache/lucene-solr/blob/df874432b9a17b547acb24a01d3491839e6a6b69/solr/core/src/java/org/apache/solr/response/DocsStreamer.java#L182 

I am a bit puzzled why we need to iterate through all the fields in the document. Why can’t we just iterate through the requested field list?    

https://github.com/apache/lucene-solr/blob/df874432b9a17b547acb24a01d3491839e6a6b69/solr/core/src/java/org/apache/solr/response/DocsStreamer.java#L156 

e.g. when pass in the field list as 

sdoc = convertLuceneDocToSolrDoc(doc, rctx.getSearcher().getSchema(), fnames)

and just iterate through fnames,  there is a significant performance boost in our case.