Add note about caching/speed in filter vs. query

docs/queries.rst

@@ -170,11 +170,13 @@ things are very different.
 A filter determines whether a document is in the results set or
 not. If you do a term filter on whether field `foo` has value `bar`,
 then the result set ONLY has documents where `foo` has value `bar`.
+Filters are fast and filter results are cached in ElasticSearch.
 
 A query affects the score for a document. If you do a term query on
 whether field `foo` has value `bar`, then the result set will score
 documents where the query holds true higher than documents where the
-query does not hold true.
+query does not hold true. Queries are slower than filters and
+query results are not cached in ElasticSearch.
 
 The other place where this affects things is when you specify
 facets. See :ref:`queries-chapter-facets-section` for details.