Why is the server slowing down?

Question

CentOS

HP proliant gen8.
Centos 7
8gb, 500gb RAID1
Installed ispmanager, used as a web server.
Lately, I've started to freak out:

• Python scripts are executed for several minutes, although previously it took no more than a minute for the same script.
  
• Mail is sent in about a minute (roundcube)
  
• PHP scripts (especially when generating pdf files) take just as long.
  

What could be the problem? I checked the disks, they seem to be working. In which direction should you dig?

Answer 1

Set monitoring and watch. For starters, you can look at htop, iftop, iotop and check the SMART on the disks and the RAID status