(One of the product managers on the Amazon EFS team here). We have many customers that use EFS for a wide variety of use cases, including hosting Postfix. As with all applications, performance needs are relative. Use EFS if your application requires consistent low single digit ms latencies, shared POSIX file system, and a pay as you go elastic usage model. As with all AWS services, EFS is always launching greater performance capabilities, including IOPS, throughput and lower latencies to meet the needs of our customers. As example, on 4/1, EFS launched a 400% improvement in read IOPS for its General Purpose performance mode, from 7,000 to 35,000. Given the type of file system operations that Postfix performs, it should nicely benefit from this improvement.