Same server, same query, different response time

I have a development server that has some problem to access the data, user reporting that it's too much slow sometimes. The setup is: * virtual server; * 4 virtual CPU; * 8 GB of virtual memory ; * 80 GB of virtual HD (the real HD is a SDD one), I had still 36 GB available; * OS Debian 9; * Mysql 5.6.47; To avoid all problems about network and the Web App, I simply do my queries directly on the host where Mysql is installed. I had enabled the logging of slow query, and find the slowest query. This query start from a certain table, that I report below:

CREATE TABLE MALICIOUS_TABLE (
  column_1 int(11) NOT NULL AUTO_INCREMENT,
  column_2 varchar(8) NOT NULL,
  column_3 datetime NOT NULL,
  column_4 int(11) NOT NULL,
  column_5 int(11) DEFAULT NULL,
  column_6 int(11) DEFAULT NULL,
  column_7 int(11) DEFAULT NULL,
  column_8 tinyint(1) DEFAULT NULL,
  column_9 datetime DEFAULT NULL,
  column_10 int(11) DEFAULT NULL,
  column_11 varchar(2048) DEFAULT 'column_11',
  column_12 tinyint(1) DEFAULT NULL,
  column_13 datetime DEFAULT NULL,
  column_14 tinyint(1) DEFAULT NULL,
  PRIMARY KEY (column_1),
  KEY fk_ual_aut_idx (column_2),
  KEY fk_aul_c_idx (column_4),
  KEY kf_ual_po_idx (column_5),
  KEY fk_ual_ute_idx (column_10),
  KEY column_1 (column_1),
  KEY column_2 (column_2),
  CONSTRAINT fk_aul_c FOREIGN KEY (column_4) REFERENCES t_table2 (column_4) ON DELETE NO ACTION ON UPDATE NO ACTION,
  CONSTRAINT fk_ual_aut FOREIGN KEY (column_2) REFERENCES t_tabl3 (column_2) ON DELETE NO ACTION ON UPDATE NO ACTION,
  CONSTRAINT fk_ual_po FOREIGN KEY (column_5) REFERENCES t_table4 (column_5) ON DELETE NO ACTION ON UPDATE NO ACTION,
  CONSTRAINT fk_ual_ute FOREIGN KEY (column_10) REFERENCES t_table5 (column_10) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=2357917 DEFAULT CHARSET=latin1

The table has a not so small number of records:

select count(*) from MALICIOUS_TABLE;
+----------+
| count(*) |
+----------+
|  2308414 |
+----------+
1 row in set (2,67 sec)

If I try the slowest query, always from the mysql command line on the server, every about 10 seconds, I got different response times, this is the production server, so users keep insert data:

SELECT count(*) FROM MALICIOUS_TABLE WHERE column_4 = 1 AND (column_8 is null) AND column_3 > CURDATE() - INTERVAL 30 DAY;
+----------+
| count(*) |
+----------+
|   666411 |
+----------+
1 row in set (4,39 sec)

SELECT count(*) FROM MALICIOUS_TABLE WHERE column_4 = 1 AND (column_8 is null) AND column_3 > CURDATE() - INTERVAL 30 DAY;
+----------+
| count(*) |
+----------+
|   666477 |
+----------+
1 row in set (4,94 sec)

SELECT count(*) FROM MALICIOUS_TABLE WHERE column_4 = 1 AND (column_8 is null) AND column_3 > CURDATE() - INTERVAL 30 DAY;
+----------+
| count(*) |
+----------+
|   666752 |
+----------+
1 row in set (17,02 sec)

The last attempt has a great variation of response time. At the beginning I thought that maybe indexes are the problem, I drop them and recreate them. Yet I got the huge variation of the response time. The RAM of the server it's good, still getting about 2 giga of free RAM. The query caching of Mysql it's active, and maybe the second attempt retrieve the query from the cache, and the last one no. Any suggestion of what I can check to understand the problem? The machine, the db (now I'm trying to modify query cache settings) or the table itself? Thank you in advance.