The clearest example of how low productivity under socialism matters is in agriculture. On the one hand, you have communes or on the extreme, state-owned farms and on the other hand you have privately operated or owned land. After decades of experience in all the socialist states, it's bitterly apparent that even when only a minority of land is under private production, that it outproduces the state-owned sector. There's a reason for this: state owned farmers "slack off", and peasants like working for their own profit.
Cuba is now recapitulating the process of moving towards private agriculture already experienced in China and Russia. If even Cuba, the best run and most humane of all Marxist-Leninist states is moving towards private plots of land, and is finding them more productive, then there are lessons should be drawn. Unfortunately, some are ideologically inoculated against learning anything or changing their minds.