Additional to what @dtech has said, let me also bring home the point that QuickBox does not control and/or setup your systems Swap and Cache. This is something that is purely determined on the user-end of the scope.
Pay attention to 'Real Memory' as the Cached Memory and Physical Memory are normal to show as high usage. They are only a reflection of the actual memory usage. Real Memory is the actual amount of memory being used on your system.
If your cpu load is high and you have around 20 users on your system, it is quite possible that your system has become slightly saturated. Again, this is something you will have to monitor on your own accord to manage these users processes, send warnings and/or kill the processes on offending users. In most circumstances it will involve several users unraring simultaneously that will cause extremely high loads.
To further give my own opinion. Having 10TB of space is well and good, and though your specs are not by any means lacking... you will need to have a considerably higher rated hardware spec to run that many users. I would honestly think about looking into another server and moving users accordingly.