Not sure what you mean by "safe" by default. Speed and performance have nothing to do with security, if that's what you mean.
The reason that the default Postgres configuration does not perform well is that Postgres runs on a number of operating systems, and it is not possible to make assumptions about the hardware and configuration of the host. For example, increasing configuration parameters like shared_buffers to any useful value will require you to tweak the OS'es kernel resources. The Postgres documentation does a great job at describing the process on a number of operating systems:
Plenty of software manages to share information between processes without using SysV shared memory, and therefore being subject to these kernel resource limits.
I'm not saying this should necessarily be fixed - I'm sure the Postgres team has better things to be working on.
But it's not a natural, unavoidable limitation, either.
The reason that the default Postgres configuration does not perform well is that Postgres runs on a number of operating systems, and it is not possible to make assumptions about the hardware and configuration of the host. For example, increasing configuration parameters like shared_buffers to any useful value will require you to tweak the OS'es kernel resources. The Postgres documentation does a great job at describing the process on a number of operating systems:
http://www.postgresql.org/docs/current/interactive/kernel-re...