Two factors determine a pulley’s efficiency:

sheave size (the wheel that the rope runs on): the larger the diameter, the greater the efficiency
bushings and bearings: self-lubricating bushings are efficient. Sealed ball bearings are very efficient and require very little maintenance.
All Petzl pulleys may be installed anywhere on the rope - even when loaded - without the need for threading an end of the rope through the pulley. Pulleys with fixed side plates are easy and quick to install on a rope, but a symmetrically shaped carabiner (eg. the OK) must be used. Pulleys with swinging side plates must be opened to load the rope. They work with all carabiner types.