Are you using the latest Google Chrome browser? The game launches in Safari as it's also webkit, but currently performance difference can be colossal between the two.
I tested it on a similar Macbook Pro as well as a really old iMac (can't remember the specs exactly but way lower than yours). On the old iMac it worked perfectly as long as Google Chrome was used - Safari performed really bad.
The lowest spec computer I used to run the game was and old, single core Pentium 4 PC which still hangs around in the office, with some shitty AGP graphics card (yep!;), and it was fine as long as Chrome was used.
I know there are other platforms and browsers out there, so I'll look into that in the future but currently I am focusing on the gameplay and the browser which happened to work best during my tests.
The other thing which could affect the performance would be if you had some Flash streaming in the other tab or another active tab with a js heavy site / app, as well as some heavy system process running.
And if the above is not the case, and you are indeed using new Chrome... than that gives me something to think about:)
Please let me know how you're getting on