100w测试数据，为什么加了索引查询反而变慢了？

建表：
create table tb_test(fval varchar(50));
----------------------------------------------
插入测试数据：
DELIMITER $$

CREATE DEFINER=`root`@`localhost` PROCEDURE `P_teset`()
BEGIN
DECLARE v_val VARCHAR(20);
DECLARE v_str VARCHAR(20);
DECLARE v_i INT ;
DECLARE v_j INT;

SET v_str ='abcdefghijklmnopqrstuvwxyz';
SET v_i=0;
SET v_j=0;

WHILE v_i<600000 DO
SET v_val ='';

WHILE v_j< 13 DO
SET v_val= CONCAT(v_val,SUBSTRING(v_str,1,FLOOR(1+RAND()*26)));
SET v_j=v_j+1;
END WHILE;
SET v_j=0;
INSERT INTO tb_test(fval)
VALUES(v_val);

SET v_i =v_i+1;
END WHILE;
END$$

DELIMITER ;
----------------------------------------------
测试like：

SELECT *
FROM tb_test
WHERE fval LIKE '%ab';

70231条 用时0.531s

SELECT *
FROM tb_test
WHERE fval LIKE 'ab%'

961406条用时1.422s
---------------------------------------------
加索引：
ALTER TABLE tb_test ADD INDEX my_index(fval);
---------------------------------------------
再测试：
SELECT *
FROM tb_test
WHERE fval LIKE '%ab';

70231条 用时1.094s

SELECT *
FROM tb_test
WHERE fval LIKE 'ab%'

961406条用时1.485s

表为MyISAM格式。

为什么加了索引却慢了？