Redmine is very slow for every page

We are using redmine on Debian 7 and we are experiencing very slow response times when navigating through it.

Hardware configuration:

• RAM: 8Go
• Proc: 4 Proc, 2800 MHz, cache size: 2048 KB

Software configuration:

MySQL

• version: 5.5.43
• innodb_buffer_pool_size: 4Go
• No slow requests detected (seconds thresholds set to 3)

ruby

• Version: 1.9.3p547

Passenger

• Version: 5.0.6
• Passenger grabs CPU log while navigating Redmine

Performance analysis I did and results:

MySQL Tunner

• do nothing after running the script

Munin

• MySQL threads avg: 3.2

We recently migrated redmine from a server that was running Centos 5.5 with the same ROR environment, with the same boot, better performance than Debian 7.

Do you have any suggestions / directions that I could explore in more depth?

Thank you for your help.

Greetings