But you can copyright the most efficient implementations.

As mentioned above, some algorithms are so simple that there's only one canonical way to write them. (Take 'looping through an array' for example - in each language there's generally one canonical way to do it.) You could argue that the text of that canonical way is copyrightable.

(It's similar to the early days of heat engines, where a guy called James Pickard managed to patent the crank, preventing other engine manufacturers from using cranks and forcing them to use a sun-and-planet gear arrangement instead: https://en.wikipedia.org/wiki/Sun_and_planet_gear )