[lhoursquentin]

Regarding 4. it's an interesting point, and this actually exists, few people know that ksh93 (which was a big inspiration for bash and zsh) does not fork when it doesn't really need to, for instance if you strace the following:

echo $(echo $(echo echo))

Due to those command substitutions, on zsh/bash/mksh/dash/yash/busybox/osh you'll see 2 forks/clones, but on ksh93 there will be none, and this allows ksh93 to be insanely fast on "pure" shell scripts that have little use of external binaries and pipes, even faster than dash which is usually known to outperform most shells.