It's being presumed in a few Q&A and discussions here that democracy is the most optimal political system. I'd like to see whether this assertion can be substantiated or refuted here.
Are their historical trends in the circumstances that would favour one system over another for economic performance?
For example, it has been argued that Soviets defeated Germany in the fields of WWII, something that French or British couldn't do until 1944, because Stalin's violent dictatorship led to single-minded mobilization of all resources. On the other hand, in the times of plenty the opportunities for diverse free enterprise implied by democracy would lead to better economic performance in a democratic society.
I'm looking for trends such as above: historically, under what circumstances would a tight oligarchy tend to outperform, and under what circumstances a democracy would?
Notice that I'm not asking for opinions such as "which system is better". I'm looking for measurable performance - such as the health of economy in the times of piece or prevailing in the times of war.