Using physics can save a lot of time when used right. A couple things to remember when using physics are: Objects with high polygon counts are going to take a long time to calculate. Try using objects with low polygon counts. You can also use an object with a low polygon count and then use the track tool to make it appear that the physics are affecting the object with a higher polygon count. Another option is to only do your physics calculation once and then disable physics. This is easy to do. Just select your scene and uncheck the "enable physics" box. This will stop you from having to wait through numerous recalculations of the physics and save a lot of time. You can also take the physics and convert it to other motion methods. When you change the motion type Carrara can create keyframes that matched the previous movement. This is good to do when you would like to save the movement of your physics object.
Carrara 8 supports Bullet Physics, an accelerated physics library, which speeds up calculations greatly.