Hi,
Atomic is made like a video game, is refreshing permently his screen even if nothing change on the screen.
The problem is not the CPU it consume, the problem is : is it slow or not?
Don't worry about CPU consumption while Atomic is in foreground. When Atomic is running a game or emulator its own CPU consumption go back to 0%. Atomic sleeps , and let all the power to the game.
Now, if you find your layout reacts too slow (it should not) , i can work with you to fix that. I use more sophisticated layout on old celeron 500mhz...
FYI, if you use a multi-cpu or hyperthreaded , the CPU usage won't be 100% of course. On machine i tested it uses around 40%.
So except if you really have performance problem don't worry about the CPU usage of Atomic.