Recursive CTE performance

WITH RECURSIVE x(i) AS (

    SELECT CAST(0 AS DECIMAL(13, 10))

     UNION ALL

    SELECT i + 1

      FROM x

     WHERE i < 101

),

Z(Ix, Iy, Cx, Cy, X, Y, I) AS (

    SELECT Ix, Iy, X, Y, X, Y, 0

      FROM (SELECT CAST(-2.2 + 0.031 * i AS DECIMAL(13, 10)) AS X,

                  i AS Ix FROM x) AS xgen

           CROSS JOIN (

               SELECT CAST(-1.5 + 0.031 * i AS DECIMAL(13, 10)) AS Y,

                      i AS iY FROM x

           ) AS ygen

    UNION ALL

    SELECT Ix, Iy, Cx, Cy,

           CAST(X * X - Y * Y + Cx AS DECIMAL(13, 10)) AS X,

           CAST(Y * X * 2 + Cy AS DECIMAL(13, 10)), I + 1

      FROM Z

     WHERE X * X + Y * Y < 16.0

           AND I < 27

),

Zt (Ix, Iy, I) AS (

    SELECT Ix, Iy, MAX(I) AS I

      FROM Z

     GROUP BY Iy, Ix

     ORDER BY Iy, Ix

)

SELECT GROUP_CONCAT(

           SUBSTRING(

               ' .,,,-----++++%%%%@@@@#### ',

               GREATEST(I, 1),

               1

           ) ORDER BY Ix SEPARATOR ''

       ) AS 'Mandelbrot Set'

  FROM Zt

 GROUP BY Iy

 ORDER BY Iy;