Hello. Yes, you're right, 7000 customers may consume large amount of resources.
Solution would be adding indexes to database. I noticed that virtuemart don't have indexes on users's table, so adding them may improve dramatically search time.
Table (prefix)_virtuemart_userinfos needs add index to:
address_type
virtuemart_user_id
last_name
And then combination of fields
last_name
first_name
company
city
address_type_name
If you don't know how to accomplish this, send us your admin + mysql admin access to
pavel.machacek@artio.czTato emailová adresa je chráněna před spamboty, abyste ji viděli, povolte JavaScript
and we can check the problem for you.