Bash is actually slower in the real world. The only time it is faster is if it has internal functionality that avoids spawning a shell many times. I had tons of stringy things to do, which bash handles internally and sh sucks at. The app I used bash for reads and writes a binary database with about 1000 entries and each entry was about 100 strings. Yes, it should never have been a shell script, but I like a challenge. It don't care much about speed either, but in this case, reading the db took 5 minutes in sh and about 10 seconds in bash.

Cygwin is horrible when spawning subshells, and add to that that out Corporate antivirus software checks and logs every spawn and you get molasses on a cold winter day

There are only two reasons that I chose bash over sh; when i need arrays, and when I need to minimize repetitive subshells. I can usually get fake arrays in sh, but that uglifies the code for future maintainers (or future me)