1. Servers are locally hosted...
2. ... in a VMWare environment.
We have a test server running exactly the same CentOS and Moodle version which has no performance issues when installing or removing plugins - The test server is also running in our virtual environment.
The DB server is hosted on its own VM. Actually I just found out that our test DB server is running a newer version of Mysql so I will try to update the production server to see if the Mysql version is the bottleneck. I will get back to you when the updates have been applied.
Thank you for your help,