Sudden increase in CPU when entering Perform Mode

Hi all. I had a friend over this afternoon and his MainStage installation is doing something I've never seen before. In Edit Mode, his CPU usage idles around the 30% mark but as soon as he enters Perform Mode, the CPU usage sudden increases to between 50-60% when idling. I've been using MS for about 6 years now and have never seen anything like that happen before. Also, he's not running any virtual instruments...only some basic MainStage plugins such as compressor, channel EQ and basic verb. Essentially, MainStage is just being used as a mixer for his outboard synths and vocal mic.

To be honest, I would have expected this machine to idle at around 10-15% CPU and max out around the 30-40% mark, especially given the lack of any heavy-duty plugins - either virtual instruments or FX. But for the CPU to suddenly increase to 50-60% when Perform Mode is engaged is crazy.

My friend's MacBook Pro specs are: mid-2014 13" retina MacBook Pro, 2.6 GHz Intel Core i5, 8 GB ram, 256GB SSD running Yosemite 10.10.1. His audio interface is a Focusrite Saffire Pro 40. Audio settings in MS are:

• I/O Buffer Size = 64 samples
• No I/O Safety Buffer
• Driver Latency = 64 samples

I realize that some users may feel that a buffer of 64 may be responsible for the CPU usage but I've run incredibly complex concerts on a mid-2009 MacBook Pro 3.1 GHz, 8GB/512 SSD on Mavericks at 64 samples with much better CPU usage than my friend's nearly brand-new machine....and he's hardly running anything that should be taxing it. I've never seen CPU usage jump 30% just going from Edit Mode into Perform Mode.

Anyone have any thoughts?

Cheers