Pulleys are wheels, but they’re not frictionless. Sheaves rest on axles, bushings, and bearings that contact a pulley block. As that pulley rotates, any friction in the system will slow it down. Even small differences in pulley friction can have a massive impact on loads. But how much? What happens when efficiency drops by 5%, or 30%? And how can your system reduce pulley friction?
Rather than standard bearings (sheaves on steel axles), some pulley systems use ball bearing pulleys that minimize the contact areas—and therefore, friction. Take a look at this comparison between standard pulley blocks and low friction pulleys by Ronstan.