Optimize MySQL FULLTEXT searches to use another index
Give this a try (and let us know if it helps):

SELECT  p.*, ft.relevance
    FROM  (
        SELECT id,
               MATCH (brand, name, variation) AGAINST('something' IN BOOLEAN MODE) AS relevance
            FROM Product
            WHERE MATCH (brand, name, variation) AGAINST('something' IN BOOLEAN MODE)
          ) AS ft
    JOIN Product AS p USING(id)
    WHERE p.side_id = 7
    ORDER BY  ft.relevance DESC
    LIMIT  10;


Also, provide EXPLAIN FOMAT=JSON SELECT ....