Thanks for your answer, could you tell me where the plugin data comes from ? As the system displayed load average is showing 9.85 and regarding your answer it's probably not comming from this. The thing i'm looking for is that maybe i have something to fix or optimize somehow and i'd like an input where to start searching :).
Kindly check the free command on your machine. Please note that the site monitor does not work with the windows based server and hence the permission might be getting denied.
With Clean or Adaptable themes, grey panel but with Boost theme, it works. So investigation still in progress, but it's theme related as mentionned by Rahul.