MySQL stored procedure to generate 1E6 rows from another table - horrible performance, would not recommend

mysql_1E6_rows_sp

DELIMITER // -- This is needed so that the individual commands don't end the stored procedure
CREATE PROCEDURE insert_zaps(IN num_rows int, IN pct_shared float) -- Two input args are needed
BEGIN
  DECLARE loop_count bigint; -- Variables are initialized with a type
  DECLARE len_table bigint;
  DECLARE rand_base float;
  DECLARE rand_offset float;
  DECLARE rand_ts timestamp;
  DECLARE rand_user bigint;
  DECLARE shared_with_user bigint;
  SELECT id INTO len_table FROM test.ref_users ORDER BY id DESC LIMIT 1; -- SELECT INTO can be used
  SET loop_count = 1; -- Or, if the value is simple, simply assigned
  WHILE loop_count <= num_rows DO
    SET rand_base = RAND();
    SET rand_offset = RAND();
    SET rand_ts = TIMESTAMP(
                    FROM_UNIXTIME(
                      UNIX_TIMESTAMP(NOW()) - FLOOR(
                        0 + (
                          RAND() * 86400 * 365 * 10
                        )
                      )
                    )
                  ); -- This creates a random timestamp between now and 10 years ago
    WITH rand AS (
        SELECT 
          FLOOR(
            (
              SELECT 
                rand_base * len_table
            )
          )
      ) 
      SELECT 
        id 
      INTO rand_user
      FROM 
        test.ref_users 
      WHERE 
        id IN (TABLE rand); -- This is the CTE demonstrated earlier to determine the table length
    INSERT INTO zaps (zap_id, created_at, owned_by) VALUES (loop_count, rand_ts, rand_user);
    IF ROUND(rand_base, 1) > (1 - pct_shared) THEN -- Roughly determine the amount of shared Zaps
      SELECT CAST(FLOOR(rand_base * rand_offset * len_table) AS unsigned) INTO shared_with_user;
      UPDATE 
        zaps 
      SET 
        shared_with = JSON_ARRAY_APPEND(
          shared_with, 
          '$', 
          shared_with_user
        ) -- JSON_ARRAY_APPEND(array, key, value)
      WHERE 
        id = loop_count;
    END IF;
    SET loop_count = loop_count + 1;
  END WHILE;
  END //
  DELIMITER ;