SQL Index needed for query in antispam_callback_queue()

antispam_callback_queue() calls a query that needs an index on antispam_spam_marks.content_id.
With 50k+ nodes and no index a query such as:
SELECT COUNT(*) AS cnt FROM node n LEFT JOIN antispam_spam_marks s ON s.content_id = n.nid AND s.content_type = 'node' WHERE ( n.type IN ('type1', 'type2', 'type3', 'type4') AND (n.status = 1))
can take dozens of seconds to execute.

attached is a hook_update_N() patch to add an index to the field.