Could you check the process on Utilities -> System Monitor? Use CPU and Memory column headers to toggle highest consuming processes, and let us know what you see? Do this without Firefox or Libre, since otherwise it just freezes your machine and we don't get anywhere.
Can you also check that your settings look similar to this?
and that you are using any drivers listed under "Additional Drivers" tab (those are the recommended ones and I assume you're using one of those)?