Edd Mann (of the Three Devs & A Maybe podcast) has shared a method of session storage he worked up to help increase performance in his application. He shows how to store sessions in memory with the help of TMPFS.

Yesterday I was looking through some application logs and noticed a significant bottleneck with I/O reads in the implemented file cache. [...] This was when I found 'tmpfs', saving me from all sorts of issues relating to adding yet another application to the production stack. 'tmpfs' appears as a mounted partition on your system, however, under the hood it allocates and uses a section of physical memory (non-persistent through reboots). [...] his results in the desired speed boosts, without tampering with the application logic itself. Even better, if the mount is unsuccessful for some reason, it will safety fall-back to using the persistent hard-disk solution.

Since PHP sessions make it easy to change the "save_path" location for the data in an ini value, setup is easy. He includes the needed configuration change and the commands you'll need to mount the tmpfs partition on your local file system.