I suppose if you aren’t the subject of the threat/attack and you have hard cover, and are concealed away from the suspects view…an ankle holster is okay.
What do you base this on?
Slow? Why? It's the surprise, the evasive movement and a killing shot. No different than any other close encounter. As I said, there is a reason they are referred to as "belly guns." If you don't know how to use an ankle holster and practice a few times each time put it on, don't bother but, that's what one should do every time they strap on, slip into a pocket, etc. Empty the weapon and work your muscle memory a bit, each and every day. Without fail.
I'm not sure exactly what a "natural" draw is, especially in a totally unnatural circumstance. Most here are writing from a position of never having "been there," imagining what such a situation will be like. Facing an armed assailant, ball bat, knife, fire arm, whatever, you are at a disadvantage as you have to assess the threat while your assailant has already made a decision to do you harm. Probably made that decision before he left his abode.
I taught "officer survival" for years. I've formerly assessed hundreds of officer involved shootings and, with the great assistance of officers around the country, helped to developed methods to improve survival. I've confronted armed threats.
Six or ten feet, not close enough to touch is in the average. If I can touch the assailant I probably wouldn't need to draw a weapon. There are other responses, some where the assailant stabs or shoots himself with his/her own weapon. No, an ankle carry can be safe and fast. Just as a weapon in a pocket, without practice, can be clumsy and undependable and . . . unsafe.