SPARQL Speed up federated query

I have my own dataset and I want to perform a federated query in SPARQL. Here is the query:

PREFIX : <http://myURIsNamespace#>
PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>
PREFIX p: <http://www.wikidata.org/prop/>
PREFIX ps: <http://www.wikidata.org/prop/statement/>
PREFIX pq: <http://www.wikidata.org/prop/qualifier/>
PREFIX wdt: <http://www.wikidata.org/prop/direct/>

select * where { 
    ?bioentity :hasMutatedVersionOf ?gene .
    ?gene :partOf wd:Q430258 .

    SERVICE <https://query.wikidata.org/sparql> { 
        ?gene p:P644 ?statement; 
              wdt:P31 wd:Q7187 ;
              wdt:P703 wd:Q15978631 ;
              wdt:P1057 wd:Q430258 .
        ?statement ps:P644 ?start .
        ?statement pq:P659 wd:Q20966585 .

        ?gene p:P645 ?statement2. 
        ?statement2 ps:P645 ?end .
        ?statement2 pq:P659 wd:Q20966585 .
        FILTER (xsd:integer(?start)>21000000 && xsd:integer(?start)<30000000)  
    }

}

I run the query via graphDB SPARQL interface but it's really really slow. It takes more than a minute to return 8 records. If I split the query in two parts, they are ridiculously fast.

Query#1

select * where { 
    ?bioentity :hasMutatedVersionOf ?gene .
    ?gene :partOf wd:Q430258 .          

}

56 records in 0.1s

Query#2

select * where { 
     SERVICE <https://query.wikidata.org/sparql> { 
        ?gene p:P644 ?statement; 
              wdt:P31 wd:Q7187 ;
              wdt:P703 wd:Q15978631 ;
              wdt:P1057 wd:Q430258 .
        ?statement ps:P644 ?start .
        ?statement pq:P659 wd:Q20966585 .

        ?gene p:P645 ?statement2. 
        ?statement2 ps:P645 ?end .
        ?statement2 pq:P659 wd:Q20966585 .
        FILTER (xsd:integer(?start)>21000000 && xsd:integer(?start)<30000000)  
    }       

}

158 records in 0.5s

Why the is the federation so slow? Is there a way to optimize the performance?