The CPU usage is a very objective thing and can vary by many different factors. For me, for example on Core i5 4460, the CPU usage is 0%, while Zoiper5 is idling. But if you have a very weak processor, it might be slightly more. As overall, Zoiper is pretty lightweight.
I know it, but Core i7-4510U... In linux... 3-4% idle
Linphone - 0% idle, but sound - bad...
I'll provide this feedback to our QA team, so they can test a bit on Linux.