SELECT COUNT DISTINCT query very slow

Hello!
In my MS-SQL 2005 Express database I have a table "Asphalt" and a number of tables with information for certain asphalttests. These tables (e.g. AV_Hohlraumgehalt,...) have a column "FID_asphalt" that references the ID of the asphalt that was tested. This column accepts NULL-values.
What I want to do is to get the number of all the tests for each asphalt.
The attached query worked very well, as long as the number of entries in the test-tables was small. Now I have between 500 and 1000 entries in each test-table, and the query takes ~45 seconds to complete.
Is there any way to speed up the query? It mustn't take longer then 1-2 seconds to get the result, which I use in a C# Win-Forms program.
Please help.
Thanks a lot!
Xandolph

SELECT	Asphalt.ID_asphalt,
		COUNT(DISTINCT AV_Hohlraumgehalt.ID_AV_hohlraumgehalt) AS AnzahlHohlraum, 
        COUNT(DISTINCT AV_Raumdichte.ID_AV_raumdichte) AS AnzahlRaumdichte,
		COUNT(DISTINCT AV_Triax.ID_AV_triax) AS AnzahlTriax, 
        COUNT(DISTINCT AV_UTST.ID_AV_UTST) AS AnzahlUTST,
		COUNT(DISTINCT AV_TSRST.ID_AV_TSRST) AS AnzahlTSRST, 
        COUNT(DISTINCT AV_Ermuedung4PBB.ID_AV_ermuedung_4PBB) AS AnzahlFAT4PBB, 
		COUNT(DISTINCT AV_Steifigkeit.ID_AV_steifigkeit) AS AnzahlSteifigkeit, 
		COUNT(DISTINCT AV_ErmuedungDTCPR.ID_AV_ermuedung_DTCPR) AS AnzahlFATDTCPR
FROM    Asphalt LEFT OUTER JOIN
                      AV_ErmuedungDTCPR ON Asphalt.ID_asphalt = AV_ErmuedungDTCPR.FID_asphalt LEFT OUTER JOIN
                      AV_Steifigkeit ON Asphalt.ID_asphalt = AV_Steifigkeit.FID_asphalt LEFT OUTER JOIN
                      AV_Ermuedung4PBB ON Asphalt.ID_asphalt = AV_Ermuedung4PBB.FID_asphalt LEFT OUTER JOIN
                      AV_UTST ON Asphalt.ID_asphalt = AV_UTST.FID_asphalt LEFT OUTER JOIN
                      AV_TSRST ON Asphalt.ID_asphalt = AV_TSRST.FID_asphalt LEFT OUTER JOIN
                      AV_Triax ON Asphalt.ID_asphalt = AV_Triax.FID_asphalt LEFT OUTER JOIN
                      AV_Raumdichte ON Asphalt.ID_asphalt = AV_Raumdichte.FID_asphalt LEFT OUTER JOIN
                      AV_Hohlraumgehalt ON Asphalt.ID_asphalt = AV_Hohlraumgehalt.FID_asphalt
GROUP BY Asphalt.ID_asphalt
ORDER BY Asphalt.ID_asphalt

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