Run Task Manager In Xubuntu as Superuser
Never use sudo to start graphical applications. (More infos)
This should work:
sudo xfce4-taskmanager
Then go to settings on top left and then click on "Show all processes".
But the root xfce4-taskmanager shows only the root processes... Perhaps this is a better solution to set your priorities:
Change niceness (priority) of a running process
The priority of a process will be lost if you close the process. But you can start any process with a certain priority: Do all apps start with a normal priority on the system monitor?