While I think you've been using Unix longer than I have, shell scripts are known for performing very poorly, and on PDP-11 Unix (where perhaps shell scripts were most heavily used, since Perl didn't exist yet) fork() couldn't even do copy-on-write; it had to literally copy the process's entire data segment, which in most cases also contained a copy of its code. Moving to paged machines like the VAX and especially the 68000 family made it possible to use copy-on-write, but historically speaking, Linux has often been an order of magnitude faster than most other Unices at fork(). However, I think people mostly don't use those Unices anymore. I imagine the BSDs have pretty much caught up by now.
https://news.ycombinator.com/item?id=44009754 gives some concrete details on fork() speed on current Linux: 50μs for a small process, 700μs for a regular process, 1300μs for a venti Python interpreter process, 30000–50000μs for Python interpreter creation. This is on a CPU of about 10 billion instructions per second per core, so forking costs on the order of ½–10 million instructions.
Python runs on other operating systems, like NT, where AIUI processes are rather more heavyweight.
Not all use cases of Python and Windows intersect (how much web server stuff is a Windows / IIS / SQL Server / Python stack? Probably not many, although WISP is a nice acronym), but you’ve still got to bear it in mind for people doing heavy numpy stuff on their work laptop or whatever.
