The problem is to be understood as follows. The official portable and safe method to create a process is using the fork() system call. This call however copies the process page tables and get seriously slow as the (Prolog) process is multiple giga bytes large. Alternatively, we may use vfork() which avoids copying the process space. But, the safe usage as guaranteed by the POSIX standard of vfork() is insufficient for our purposes. On practical systems your mileage may vary. Modern posix systems also provide posix_spawn(), which provides a safe and portable alternative for the fork() and exec() sequence that may be implemented using fork() or may use a fast but safe alternative. Unfortunately posix_spawn() doesn't support the option to specify the working directory for the child and we cannot use working_directory/2 as the working directory is shared between threads.

Summarizing, the default is safe and tries to be as fast as possible. On some scenarios and on some OSes it is possible to do better. It is generally a good idea to avoid using the cwd(Dir) option of process_create/3 as without we can use posix_spawn().