Mumble use on my MacBook Pro 20% CPU. My Model has a 2.4Ghz Duo Core CPU. My Desktop PC has the some CPU and the CPU Usage on Windows is under 10%.
I've noticed in general (and not just Mumble) that audio seems to use more CPU% than what's typical on Windows.
I've not looked further into this.
I have also noticed on Linux that mumble seems to use more CPU than I would expect, even when it is apparently idle the CPU usage on my Athlon X2 4800+ can be over 10%. It never causes a "problem", but it does seem un-naturally high. (This applies to 1.2.2 and 1.2.3rc1)