MySQL – Troubleshooting Slow Queries on Indexed Columns

I've got some large MySQL (5.5.60) tables where queries take 60 seconds+ to complete.

product_base (1.5GB, 26 million rows)

CREATE TABLE `product_base` (
  `EAN13` varchar(13) NOT NULL DEFAULT '',
  `company_tk` int(11) DEFAULT NULL,
  `category` int(11) DEFAULT NULL,
  `modified` varchar(30) DEFAULT NULL,
  KEY `EAN13` (`EAN13`),
  KEY `company_tk` (`company_tk`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

attribute (11.8GB, 126 million rows)

CREATE TABLE `attribute` (
  `EAN13` varchar(13) DEFAULT NULL,
  `attribute_type` int(11) DEFAULT NULL,
  `val_t` varchar(1500) DEFAULT NULL,
  `val_n` decimal(15,4) DEFAULT NULL,
  `val_d` datetime DEFAULT NULL,
  `modified` datetime DEFAULT NULL,
  KEY `EAN13` (`EAN13`),
  KEY `attribute_type` (`attribute_type`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

attribute_type (16KB, 59 rows)

CREATE TABLE `attribute_type` (
  `attribute_type` int(11) DEFAULT NULL,
  `title` varchar(90) DEFAULT NULL,
  `field_name` varchar(90) DEFAULT NULL,
  `data_type` char(3) DEFAULT NULL,
  `seq` int(11) NOT NULL,
  `html` int(11) DEFAULT NULL,
  KEY `attribute_type` (`attribute_type`),
  KEY `seq` (`seq`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

Example slow query:

Query_time: 61.629684  Lock_time: 0.000138 Rows_sent: 1  Rows_examined: 28141979
select p.EAN13, p.company_tk, a.attribute_type, a.val_t, t.title, t.field_name from product_base p
inner join attribute a on p.EAN13=a.EAN13 
inner join attribute_type t on a.attribute_type=t.attribute_type 
where p.EAN13=705632109762 and (a.attribute_type=1 or a.attribute_type=3)
order by t.seq;

Running explain on that query:

+----+-------------+-------+-------+----------------------+----------------+---------+--------------------------+---------+-----------------------------+
| id | select_type | table | type  | possible_keys        | key            | key_len | ref                      | rows    | Extra                       |
+----+-------------+-------+-------+----------------------+----------------+---------+--------------------------+---------+-----------------------------+
|  1 | SIMPLE      | t     | range | attribute_type       | attribute_type | 5       | NULL                     |       2 | Using where; Using filesort |
|  1 | SIMPLE      | a     | ref   | EAN13,attribute_type | attribute_type | 5       | eandata.t.attribute_type | 6299937 | Using where                 |
|  1 | SIMPLE      | p     | ref   | EAN13                | EAN13          | 54      | eandata.a.EAN13          |       1 |                             |
+----+-------------+-------+-------+----------------------+----------------+---------+--------------------------+---------+-----------------------------+

Server/OS:
Debian 8 64bit | 24GB RAM (MySQL is using 70% of this) | 60GB SSD | 2xIntel Xeon CPU E5506 @ 2.13GHz | MySQL 5.5.60

Innodb settings here

MySQL my.cnf

# * Fine Tuning
key_buffer      = 16M
max_allowed_packet  = 16M
thread_stack        = 192K
thread_cache_size       = 8
# This replaces the startup script and checks MyISAM tables if needed
# the first time they are touched
myisam-recover-options  = BACKUP
max_connections        = 20
table_cache            = 9552
thread_cache = 10
sort_buffer_size = 512K
net_buffer_length = 8K
read_buffer_size = 256K
read_rnd_buffer_size = 512K
myisam_sort_buffer_size = 16M
query_cache_size = 512M
join_buffer_size = 2G
innodb_buffer_pool_size = 15G
innodb_io_capacity = 2000
innodb_read_io_threads = 64
innodb_thread_concurrency = 0
innodb_write_io_threads = 64
#innodb_log_file_size = 512M
innodb_flush_log_at_trx_commit = 1
innodb_flush_method = O_DIRECT
innodb_buffer_pool_instances = 8

max_heap_table_size = 2G
tmp_table_size = 2G
wait_timeout = 120
interactive_timeout = 120

Any suggestions for the slow queries?