Oracle version 7.0 on AIX 3 uses the OS-supported locks on the shared memory segments and allows the application designer to link an application in a so called single tasking way. Instead of starting one client process per application process both the database part and the application part is run in the same process. In later versions of Oracle this is no longer supported.
This would require a total redesign of the Postgres access model and that effort can not justify the performance gained.