Why is the "ci_sessions" table type MyISAM?

[eluser]RaZoR LeGaCy[/eluser]
Running APC plus memcache.

I am having a lot of slow queries for ci_sessions, the list is longer but you get the idea. MySQL log returns:

Code:

# Time: 100727 18:51:45

# Query_time: 14  Lock_time: 0  Rows_sent: 0  Rows_examined: 0
INSERT INTO `ci_sessions` (`session_id`, `ip_address`, `user_agent`, `last_activity`) VALUES ('e27c96e05f2a22bf87b77ebb5aac2b2d', '67.195.114.215', 'Mozilla/5.0 (compatible; Yahoo! Slurp/3.0; http://', 1280271091);
# Time: 100727 18:53:10

# Query_time: 12  Lock_time: 0  Rows_sent: 0  Rows_examined: 0
INSERT INTO `ci_sessions` (`session_id`, `ip_address`, `user_agent`, `last_activity`) VALUES ('664d9ed5da1e7086dbce48bf48fc48c8', '118.81.99.85', 'Mozilla/3.0 (compatible; Indy Library)', 1280271178);
# Time: 100727 19:08:53

# Query_time: 12  Lock_time: 0  Rows_sent: 0  Rows_examined: 0
INSERT INTO `ci_sessions` (`session_id`, `ip_address`, `user_agent`, `last_activity`) VALUES ('094498eaed248968bad4fa045190b2ae', '66.249.65.145', 'Mozilla/5.0 (compatible; Googlebot/2.1; +http://ww', 1280272121);
# Time: 100727 19:14:13

# Query_time: 13  Lock_time: 0  Rows_sent: 0  Rows_examined: 0
INSERT INTO `ci_sessions` (`session_id`, `ip_address`, `user_agent`, `last_activity`) VALUES ('229c467afb6bb42b9f0bceaef8155542', '66.249.65.145', 'Mozilla/5.0 (compatible; Googlebot/2.1; +http://ww', 1280272440);
# Time: 100727 19:15:20

# Query_time: 14  Lock_time: 0  Rows_sent: 0  Rows_examined: 0
INSERT INTO `ci_sessions` (`session_id`, `ip_address`, `user_agent`, `last_activity`) VALUES ('d2a7795692eda11379ca56247d4f675f', '67.195.114.215', 'Mozilla/5.0 (compatible; Yahoo! Slurp/3.0; http://', 1280272506);
# Time: 100727 19:47:55

# Query_time: 12  Lock_time: 0  Rows_sent: 0  Rows_examined: 0
use ci;
INSERT INTO `ci_sessions` (`session_id`, `ip_address`, `user_agent`, `last_activity`) VALUES ('7b808ec0c2de270abc78adbe2c74350e', '66.249.65.220', 'Mozilla/5.0 (compatible; Googlebot/2.1; +http://ww', 1280274463);

DB Structure

Code:

CREATE TABLE IF NOT EXISTS `ci_sessions` (
  `session_id` varchar(40) collate utf8_unicode_ci NOT NULL default '0',
  `ip_address` varchar(16) collate utf8_unicode_ci NOT NULL default '0',
  `user_agent` varchar(50) collate utf8_unicode_ci NOT NULL default '',
  `last_activity` int(10) unsigned NOT NULL default '0',
  `session_data` text collate utf8_unicode_ci,
  `user_data` text collate utf8_unicode_ci,
  PRIMARY KEY  (`session_id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

config/database.php

Code:

$db['default']['pconnect'] = TRUE;
$db['default']['db_debug'] = TRUE;
$db['default']['cache_on'] = FALSE;
$db['default']['cachedir'] = "cache";

config/config.php

Code:

$config['sess_expiration']      = 7200; // 2hours

$config['sess_encrypt_cookie']  = TRUE;

$config['sess_use_database']    = TRUE;

$config['sess_table_name']      = 'ci_sessions';

$config['sess_match_ip']        = FALSE;
$config['sess_match_useragent'] = FALSE;

$config['sess_time_to_update'] = 300;

Seems like Yahoo/Google bots are the culprits but soon users will be seeing these issues when site traffic increases.

What are some solutions to fix this?