I see the same problem om my system - a couple of Evernote processes in the task manager and one of them running at 5-10% CPU.
Apparently it has something to do with the note content, because the problem completely disappears when selecting a note with empty content ... and if I select a note with a decent amount of content, the CPU goes up again.
After that I tried opening a couple of different notes in different windows. Now the process count goes up - one for each window - but all the processes idle at 0% CPU. What?
Closing all the windows again and the CPU is still at 0%.
Now I go back to the initial note and let that have focus aging - and - voila - the CPU goes up again.