Getting list of creators / publishers is slow

[coret]

The search form in the demonstrator has pulldown lists to select a creator and/or publisher. This lists are populated via the SPARQL query:

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX dcat: <http://www.w3.org/ns/dcat#>
PREFIX dct: <http://purl.org/dc/terms/>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>

SELECT DISTINCT ?creator ?creator_name WHERE {
	?dataset a dcat:Dataset .
	?dataset dct:creator ?creator .
	?creator foaf:name ?creator_name
	FILTER isIRI(?creator) 
	FILTER(LANG(?creator_name) = "" || LANGMATCHES(LANG(?creator_name), "'.$lang.'")) 
	BIND(LCASE(STRDT(STR(?creator_name), xsd:string)) AS ?creator_name2)
} ORDER BY ?creator_name2

This query takes 1.8s ?!? Although the results are cached, these queries must be optimized.

[coret]

The query with parts commented out has the same responsetime (so the optimization part isn't to be found in the commented out part):

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX dcat: <http://www.w3.org/ns/dcat#>
PREFIX dct: <http://purl.org/dc/terms/>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>
SELECT DISTINCT ?creator ?creator_name WHERE {
#   ?dataset a dcat:Dataset .
    ?dataset dct:creator ?creator .
    ?creator foaf:name ?creator_name .
#   FILTER isIRI(?creator) 
#   FILTER(LANG(?creator_name) = "" || LANGMATCHES(LANG(?creator_name), "nl")) 
#   BIND(LCASE(STRDT(STR(?creator_name), xsd:string)) AS ?creator_name2)
}
#ORDER BY ?creator_name2