I'm assuming your talking about Vector Motion Blur. I also am guessing that the clouds are contributing to an extended render time. The real clouds, especially if you have some environment lighting enabled, take some time

Motion blur is cool but it's a mixed bag because it does add to the render time. It's great for stills and good for animation. The temptation is to "over set" the parameters which can create render times of days instead of hours. Fundamentally, motion blur is for destroying detail in the image. For an animation, confining the blur to two or three frames is best. For stills I'll usually use a blur of ten frames.

It's important to understand how motion blur works. If you set it with an "Extra Frame" value of 5 and enable Before and After, for every "real" frame in your scene, Carrara will actually render 11 frames - 5 before, 5 after plus the real frame and then it composites them together. So, using your example numbers, an animation with only 168 real frames might actually be tasking Carrara to render 1168 frames. If your average frame render time is 5 minutes per frame, there's your hour: 11x5 minutes or 55 minutes per 'real' frame.

The faster your object is moving in scene, the less Pre and Post frames are needed for animation. Since it's a moving, detail destroyed object, the eye fills in the missing detail - you don't really need to render it out.

However, for stills, since there is no actual visible motion to hide low fidelity, a greater frame blurring number is warranted. 

Why would you use Carrara's motion blur at all? Because it also renders the blur in reflections and through transparency - something that's very difficult to do, even in post production for animations. For stills you can cheat a little and use Photoshop.

There's a reason that in animation studios, all the 3D work is done during the day and then everything is sent to the render farm overnight.

Mark