Weird, does not seem to be changing anything.
I go shift-O, paste preferences.afm_time = 1 for example and the game has the character speaking say : "preferences.afm_time"
And the speed seems unchanged. I tried 1 and 50 and they are the same. I did not time it specifically but I would expect a big difference there.
Guess the game just does not allow it for some reason, which is why the slider is missing from the menu.