Chip Brown, one of my Schizoid team-mates, and I sat down with his Mac all yesterday afternoon narrowing in on the problem. We found a problem, and it seems to be related to the browser text, or perhaps the gateway from NaCl to the browser -- it's quite possible that my previous article on browser text for games is bad advice -- but had to go home to our families before we'd completely solved it.
The disparity in framerate between Windows machines and Macs does seem to highlight a truth: native client isn't magic. I still have to do compatibility testing on various platforms. If I had a Mac and Linux machine handy I would have caught these problems as I introduced them.
On the other hand, I'm trying to spend as little possible on this project. I've spent far more than I've made on indie development (especially if you count the opportunity cost of what I could have made working for The Man) and am trying to follow the advice of an indie role-playing game developer (but I forget who): don't spend any money until you make some. Wait for that first pre-order, and then print your first book. Print one copy. When you've made enough money to do a small print run, then do a small print run. Etcetera.
A friend suggested that I go to an Apple Store and do my development there. Maybe they'll think it's cool. So that's my tentative plan for today.
Though if you're a Mac owner You Can Help - get in touch with me via Google+ or Twitter (jdfristrom) - I'll let you know when I have new versions up on a test site that you can try for me. I'll give you credit and in-game energy and, I don't know, give your name on the leaderboards a different color.