Hello,
I just have a VPS and I don't have access to anything but my VPS RDP.
I asked the support and they said:
Your vps itself consuming resources of main server. Please check attached screen shot.
You will find that it is running most of the nppool memory and that caused some slowness
while accessing plesk.
Please optimize your scripts and databases with the help of your web developer.
But I restarted the SQL and w3 services and know my server just have 600M memory in use (it has 2GB of memory).
I attached the picture my admin sent and also the screen shot I taken after restarting the services.
Thanks for your attention
Iman