Does seem like sometimes the studio especially the ones that make action movies like to have stunt man in wig for the role of henchwoman. I guess maybe because a stunt man can do more dangerous stunt than an stunt woman but that is just me thinking.
You are absolutely correct. Actually there is shortage of skillful female stunt woman.
Also you don't need to "watch in slow motion" to find if the stunts are men or women. They can cheat on the breasts, but they can't cheat on the waist. Also the way they fight is a lot more musculan than feminine.
Its always fascinating to see stunt man standing in for a woman character.