You may or may not have been aware that up until 10.0.6 there were large differences between render times of effects on the timeline and the same effects on export. This was because on export, the Mac used all the resources it could to get that movie out and that included the GPU or Graphics Processing Unit. Rendering on the timeline was a different matter and that was done by the CPUs in the background. In the 10.0.6 update, the GPU is now used for rendering on the timeline which has led to a massive improvement in rendering times.
Apple is playing catchup to Adobe’s Mercury Playback Engine in Premiere Pro CS6, but it’s still a very impressive improvement.