I would not worry too much over this. I have MenuMeters even before I was introduced to smalltalk over a year ago and Pharo is steadily reported consuming a 5% of my CPU which is average for an ObjC app on Macos 10.7
So personally I am impressed with pharo speed so far. A big thumbs up from me. Also take into account that even though MacOS is a great OS it comes with its own fair share of bugs, especially the first year of its release so I would not put too much attention into this if I was you.
Now with Mavericks we can tell that Pharo is hungry for energy :)
<Screenshot 2013-10-28 17.05.48.png>
Cheers!
Uko