Access exporting to CSV is weirdly slow I have searched but could not find anything similar. If I just run the query, I can get the result pretty quickly (~5 mins); but after I saved the query to export as CSV, the exporting part is extremely slow (>30mins). 

The result only has 3 columns and approximately 30 records; I can easily copy and paste the query result I get. However, I would like to understand the strange behaviour. It's a huge database of about 7GB, that's why it's split into 4 different files. I am not allowed to use other software such as POSTgreSQL or mySQL at work. Here is my code anyway (modified the variable names):

SELECT var1, var2, count(*)
FROM (
SELECT var1,var2
    FROM [XXX10_2006-2010] INNER JOIN [XXXLong_2006-2010] 
        ON [XXX10_2006-2010].ID = [XXXLong_2006-2010].id
    WHERE [XXXLong_2006-2010].var4 = 'hamburger' 
        AND [XXXLong_2006-2010].cat1 = 'c'
        AND [XXXLong_2006-2010].cat2 BETWEEN 0 AND 96
        AND [XXX10_2006-2010].var5 = 'eggs'
UNION all 
SELECT var1,var2
    FROM [XXX10data_2001-2005] INNER JOIN [XXXLong_2001-2005] 
        ON [XXX10data_2001-2005].ID = [XXXLong_2001-2005].id
    WHERE [XXXLong_2001-2005].var4 = 'hamburger' 
        AND [XXXLong_2001-2005].cat1 = 'c'
        AND [XXXLong_2001-2005].cat2 BETWEEN 0 AND 96
        AND [XXX10data_2001-2005].var5 = 'eggs'
UNION all 
SELECT var1,var2
    FROM [XXX9_data_1997-2000] INNER JOIN [XXXLong_1997-2000] 
        ON [XXX9_data_1997-2000].ID = [XXXLong_1997-2000].id
    WHERE [XXXLong_1997-2000].var4 = 'hamburger' 
        AND ([XXXLong_1997-2000].cat1 ='1' AND [XXXLong_1997-2000].cat2 BETWEEN 40 AND 99)
        OR ([XXXLong_1997-2000].cat1 ='2' AND [XXXLong_1997-2000].cat2 BETWEEN 0 AND 39)
        AND [XXX9_data_1997-2000].var5 = 'eggs'
UNION all 
SELECT var1,var2
    FROM [XXX9_data_1993-1996] INNER JOIN [XXXLong_1993-1996] 
        ON [XXX9_data_1993-1996].ID = [XXXLong_1993-1996].id
    WHERE [XXXLong_1993-1996].var4 = 'hamburger' 
        AND ([XXXLong_1993-1996].cat1 ='1' AND [XXXLong_1993-1996].cat2 BETWEEN 40 AND 99)
        OR ([XXXLong_1993-1996].cat1 ='2' AND [XXXLong_1993-1996].cat2 BETWEEN 0 AND 39)
        AND [XXX9_data_1993-1996].var5 = 'eggs'
)
GROUP BY var1, var2


EDIT: In case this little bit of detail helps: It takes slightly more than 10 mins for the export text wizard to show up, then the rest of the process takes another 20 mins or so. (sometimes much longer though)
Off the top of my head I might think it to be related having 4 different DB files and large ones at that (not sure why that would be). Can you easily scope the query to hit 1 vs 4 DB files to see if the preformance is increased?