By "efficient" I mean specifically decision-making efficiency. In a feudal system you do not need to gain consensus from a large number of people, which means that you can adapt to changing conditions faster and gain an edge on the battlefield.

I agree that it has less productive efficiency - democracies usually have stronger economies, often much stronger, than feudal societies. I suspect that this is actually related to the previous point. The increased decision-making efficiency of feudalism comes from a reduced need to get buy-in from people, but if you don't have buy-in from the people doing the work, they will probably half-ass the work. You see this throughout history: the serf works less than the freeholding peasant, the feudal society does not innovate, new inventions get shot down by the social hierarchy, industrialism does not take hold or when it does it's in inefficient top-down forms, etc.

There's still an unresolved contradiction here in that this would imply that feudalism would be more successful in times of quick change, but the historical record is that feudalism becomes very entrenched during times of stasis or decline, but often gets outcompeted in times of rapid growth and innovation. I still have no idea what's up here; perhaps it has to do with the existence of feudal-structure organizations (eg. corporations) within a democratic framework.

I think feudalism attracts people who fear systemic corruption as feudalism is based on the naked self interest of a very limited number of people.

Normally, that would also make it easier to model, but feudalism also tends to gain entropy through inbreeding.