By default, the Postfix spooler use the hard disk to play with messages.
If you need to speedup the Postfix process you can dedicate a part of your server RAM has the spool directory.
Tthis method will improve the Postfix performance but understand that you will lose mails if the server is rebooted or is turned off.
Use this feature if you know that mails are processed without need to store them for a long time.
