This simple mysql query is taking 2-3 seconds
Lots of simplification and speedup:
SELECT e.*
    FROM ( SELECT DISTINCT event_id
             FROM events_tags
             WHERE tag_id = 617 ) AS et
    JOIN events  AS e  ON e.id = et.event_id
ORDER BY ???  -- else get random 50??
LIMIT 50;

events_tags smells like a many:many mapping table.  It can possibly be improved by following these tips.