If the fields you're using to join the tables are each indexes or primary keys, it already knows which records have matching values. Without indexes, it has to scan through the whole table(s), matching up values.
Foreign Key does not automatically create an index: http://stackoverflow.com/questions/836167/does-a-foreign-key-automatically-create-an-index
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If the fields you're using to join the tables are each indexes or primary keys, it already knows which records have matching values. Without indexes, it has to scan through the whole table(s), matching up values.
Foreign Key does not automatically create an index:
http://stackoverflow.com/questions/836167/does-a-foreign-key-automatically-create-an-index