Datatype keyword generates sql request like this one.

SELECT ezkeyword.keyword
FROM ezkeyword_attribute_link, ezkeyword
WHERE ezkeyword_attribute_link.keyword_id = ezkeyword.id
AND ezkeyword_attribute_link.objectattribute_id = "my_id"

Without index on "objectattribute_id" and with a lot of data, this request can be very slow.
When doing an EXPLAIN, you can see that without index, mysql fetch all the content of the keyword table.

Adding an index on objectattribute_id increases performance.