Serhiy Storchaka: "If this is an optimization, what is the downside of always using vfork()?"

I don't know the vfork() function, but you can find articles like:
"vfork considered dangerous" (old article of 2012)
https://ewontfix.com/7/

But it's unclear to me if vfork() drawbacks also affect posix_spawn(). posix_spawn() is well defined: call vfork() and then immediately exec().

Another article:

"First is that vfork pauses the parent thread while the child executes and eventually calls an exec family function, this is a huge latency problem for applications."

https://developers.redhat.com/blog/2015/08/19/launching-helper-process-under-memory-and-latency-constraints-pthread_create-and-vfork/