How much RAM is in that MBP?
I'd make sure you have at least 15% of your hard drive capacity as free space.
In the Render menu (top right of canvas) be sure to turn off everything you're not using (shadows, etc), and lower the resolution to make playback easier for your laptop. You can change these back to full resolutoin when you export.
You can try to trash Motion's pref files quickly and safely with "Preference Manager", free from Digital Rebellion (dot com).
If that doesn't work, move the Motion app from the Applications folder to the Trash. Launch the App Store, in the Purchased tab, click Install for Motion.
But be sure that MBP is maxed out on RAM for stuff like Motion.
I have 8GB of RAM but what's funny is that when I had Snow Leopard, my current hardware and settings in Motion were fine. I disabled all the render options when I edit like u said and even my resolution was set low before this post and while I had snow leopard. I'm beginning to lean towards Mountain Lion's fault. Im going to reinstall and see if this problem persists.