Mysql – Periodically very slow execution of simplest queries

Inspecting MySQL slow query log I found out that in certain periods of time there are a bunch of queries in log which are supposed to be very fast but were actually very slow.

For example:

# User@Host: ***[***] @  [***]
# Query_time: 14.532574  Lock_time: 0.050162 Rows_sent: 18  Rows_examined: 18
SET timestamp=1483014348;
CALL get_games();
# User@Host: ***[***] @  [***]
# Query_time: 15.287524  Lock_time: 0.008114 Rows_sent: 18  Rows_examined: 18
SET timestamp=1483014348;
CALL get_games();
# User@Host: ***[***] @  [***]
# Query_time: 15.637633  Lock_time: 0.027461 Rows_sent: 18  Rows_examined: 18
SET timestamp=1483014348;
CALL get_games();
# User@Host: ***[***] @  [***]
# Query_time: 15.070246  Lock_time: 0.050137 Rows_sent: 18  Rows_examined: 18
SET timestamp=1483014348;
CALL get_games();

Where procedure get_games is just a simple SELECT from table with 18 rows and normally executes immediately.

MySQL [db_receipt]> show create table game;
MySQL [db_receipt]> CREATE TABLE `game` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `product_id` smallint(5) unsigned NOT NULL COMMENT 'Гейтовый идентификатор игры',
  `product_name` varchar(64) COLLATE utf8_unicode_ci NOT NULL COMMENT 'Название игры для печати в квитанции',
  `arm_url` varchar(32) COLLATE utf8_unicode_ci DEFAULT NULL COMMENT 'Url в АРМ-е',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=20 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

MySQL [db_receipt]> show create procedure get_games;
+-----------+----------+----------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+--------------------+
| Procedure | sql_mode | Create Procedure                                                                                                                       | character_set_client | collation_connection | Database Collation |
+-----------+----------+----------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+--------------------+
| get_games |          | CREATE DEFINER=`receipt_adm`@`%` PROCEDURE `get_games`()
l_proc: begin

    SELECT  `product_id`, `product_name` FROM `game`;

end | utf8                 | utf8_unicode_ci      | utf8_unicode_ci    |
+-----------+----------+----------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------------------+---- 

There are also a lot of other slow simple queries such select a row by primary key etc.

What can cause such a performance degradation? How can I troubleshoot a problem to find a reason?

EDIT: MySQL Server has very good characteristics and normally queries run fast. It's a production server under high load.

EDIT2: Another slow query example (on another database), also very simple:

# Time: 161230 12:13:39
# User@Host: ***[***] @ *** [***]
# Query_time: 4.780936  Lock_time: 0.000069 Rows_sent: 0  Rows_examined: 1
use ***;
SET timestamp=1483089219;
update ShoppingCart set cartStatus='ACQUIRED' where shoppingCartId=23423;