I've got my new speed passion motor, and it's having the same problems, like it's struggling to pull away.
One thing I did think of, I did recently replace all the screws on the car for hex screws, so to counter that I've loosened them all off a little, but there's still no difference.
I don't think the car feels like the drive train is stiff.
I've uploaded a video showing the car pulling away and how it struggles, I'm also in the process of uploading another video showing the pinion taken off the motor to show how the drive train moves freely.
http://www.youtube.com/watch?v=IrehkfpeD3k