In this new post Patrick Allaert offers a solution that can help speed up the inclusion of files via the Composer autoloader (in addition to the already present "optimize-autoloader" option).

The problem with the classmap strategy and the nature of PHP is that there is no (easy) way to have a persistent variable across requests containing the classmap. [...] This [large returned array of mappings] can even take a big portion of your request's response time when you have hundreds or thousands of classes like it is the case with eZ Publish 5 being based on Symfony, where about 2 600 classes are involved.

He suggests something that could be included into the Composer functionality itself - creating symbolic links in the PSR-0 standard to the location of the files to make it easier for Composer to resolve their location (based on namespace, not having to find them). Some sample code is included showing an additional autoloader that then uses the vendor names to match the path directly.