High CPU usage even when no application is running(Desktop)
You first should figure out what process is eating away your CPU.
- open a terminal with ctrl+alt+t
- execute the command
top - note the process using the most cpu
If the process isn't a system process, kill it with sudo pkill [processname] with [processname] the name of the process you want to kill. If it is a system process, don't kill it, but try to google the name of it and figure out what functionality it does in Ubuntu. Then you can proceed to turn it off (in e.g. you system settings).
Despite you not having a program open on the desktop there are still scores of processes running on your computer. On Windows you'd use the Task Manager to see which processes are running. On Ubuntu you use the System Monitor (Applications, System Tools, System Monitor). Click on the % CPU column to order by CPU hogs first and you'll soon see which processes are at fault.
Likely candidates:
gnome-system-monitor itself because, well, you're running it and it's doing stuff.
tracker-miner-fs, which is the file indexing service. Go to Applications, System Tools, Preferences, Search and Indexing and select "Only when computer is being used." There's no "off" switch, unfortunately, but you can achieve the same result by moving the "Stop indexing when disk space is below" slider to some really high value.
compiz, which is the desktop compositor. Try going to Applications, System Tools, Preferences, CompizConfig Settings Manager and turning off all of the (probably unnecessary) eye candy.
Even with all your CPU cores pegged at 30% your CPU should not be getting overly hot. Consider clearing any dust from its heat sink and/or upgrading to a better one.