How Can I Improve The Speed Of An UPDATE Query

I have a (quite) specific question but I thing it covers something I simply cannot answer.

I have three UPDATE queries running on linked tables in Microsoft Access (2000/XP).

My main data table (the one to be updated) has almost 1million records

My three information tables ALL have primary keys (which are used to link the main table) and vary in size

I have atatched the three UPDATE queries plus descriptions of the field names used.


TableRecordsTime

Main DataTable900000
Mask nomenk1302 hours
Mask media90015 minutes
Mask brand4000?????



Query A
UPDATE [Main DataTable] AS z
INNER JOIN [Mask nomenk] AS n ON (z.nomCode1 = mn.nomCode1) AND (z.nomCode2 = mn.nomCode2) AND (z.nomCode3 = mn.nomCode3) AND (z.nomCode4 = mn.nomCode4)
SET z.NomenkMask1 = n!NomenkMask1;

Query B
UPDATE [Main DataTable] AS z
INNER JOIN [Mask media] AS mm ON (z.couCode = mm.couCode) AND (z.nomCode1 = mm.nomCode1) AND (z.pubCode = mm.pubCode)
SET z.MediaMask1 = mm!MediaMask1;

Query C
UPDATE [Main DataTable] AS z
INNER JOIN [Mask brand] AS mb ON (z.couCode = mb.couCode) AND (z.nomCode1 = mb.nomCode1) AND (z.brCode1 = mb.brCode1) AND (z.brCode2 = mb.brCode2)
SET z.BrandMask1 = mb!BrandMask1;


FieldnameFieldType
couCodeText
pubCodeText
nomCode1Long Integer
nomCode2Long Integer
nomCode3Long Integer
nomCode4Long Integer
brCode1Long Integer
brCode2Long Integer


My problem, quite simpley is the speed involved with running these queries. I know that query b) is the quickest with query a) a distant second (I could not even complete the running of query c) and killed it after 6 hours.

What I need to know is WHY is queryC soooo much slower than queryB when the only realy diference that I can see between them if the latter has an extra field to join on