Wordpress extremely slow at VPS

[Lj Belmeska]

Recently a client buy a VPS at 1&1 and I moved the Worpress site from old shared hosting (Bluehost) to new VPS. The site is extremely slow and I don't know how to fix this meaning where to attack, if I should optimize something in Apache, if I should install some plugin to avoid this or something else so I need some advice. Some data around that WP:

• Database hold around 35k posts
• I have disable W3C Total Cache plugin since I think is miss configured and can slow down the site
• I have not using any image compressor like thimbthumb (this was installed in first but due to security reasons I remove from the site)
• The VPS has two cores and 4GB of RAM and it's running Centos, PHP 5.3.3, MySQL 5.5.x and Apache 2.2.15

Can any give me some ideas? Advice? Experience? I understood what this article told me, but still: https://smartclick.mk/why-is-my-wordpress-site-so-slow/. Do not know what else to do or how to attack this problem and my knowledge does not come so far on this.

Performing Tests:

In order to find the bottleneck I've enabled:

• Wordpress Debug Logs:

  define('WP_DEBUG', true);
  define('WP_DEBUG_LOG', true);
  define('WP_DEBUG_DISPLAY', false);
  @ini_set('display_errors',0);

  This logs most of the time errors like this 

  [13-Feb-2015 12:01:46] PHP Notice: Undefined property: stdClass::$category_parent in /var/www/vhosts/elclarinweb.com/httpdocs/sistema/wp-content/themes/elclarin_v2/functions.php on line 136

   

• MySQL logs: mysql.log, mysqld.log and mysqld_slow.log

For mysql.log I've noticed a tons of access and for mysqld_slow.log I've noticed the following output:

SET timestamp=1423830131;
SELECT option_name, option_value FROM wp_options WHERE autoload = 'yes';
# User@Host: elclarin_us[elclarin_us] @ localhost []
# Query_time: 0.001520  Lock_time: 0.000043 Rows_sent: 227  Rows_examined: 261
SET timestamp=1423830131;
SELECT option_name, option_value FROM wp_options WHERE autoload = 'yes';
# Time: 150213 13:22:14
# User@Host: elclarin_us[elclarin_us] @ localhost []
# Query_time: 0.003051  Lock_time: 0.000097 Rows_sent: 227  Rows_examined: 261
SET timestamp=1423830134;
SELECT option_name, option_value FROM wp_options WHERE autoload = 'yes';
# User@Host: elclarin_us[elclarin_us] @ localhost []
# Query_time: 0.003259  Lock_time: 0.000097 Rows_sent: 227  Rows_examined: 261
SET timestamp=1423830134;
SELECT option_name, option_value FROM wp_options WHERE autoload = 'yes';
# Time: 150213 13:22:15
# User@Host: elclarin_us[elclarin_us] @ localhost []
# Query_time: 0.001597  Lock_time: 0.000048 Rows_sent: 227  Rows_examined: 261

Why the same query is executed all the time? Any web server thread is killing my Wordpress?