Why use a PSR-0 or PSR-4 autoload in composer if classmap is actually faster?

I understand that you can use either a PSR standard to locate files, or tell composer a directory to scan for classes. The documentation recommends using the PSR-4 standard. There is also an option for composer to create an optimized autoloader, which basically generates a full classmap. So why should one use PSR-4 at all if the best way to load is with a classmap?