Flaming doesn't belong to my duties
Anyway, that Boko guy is right.
The class which handles autotiles is the Tilemap
class, which is a built-in RGSS class (i.e. hidden and written in Ruby + C, or something similar).
I feel you won't be able to speed them up, I'm sorry.
Autotiles can't be managed directly from the script editor, as a result you'll need some custom script.
I'll browse Google to see if someone has managed to write a custom RGSS version of it, but I can't promise anything.