Are you performing list calls sequentially? If you have O(100k) directories and are doing O(100k) requests sequentially, 15 minutes works out at O(10ms) per request which doesn’t seem that bad? (assuming my math is correct…)
At risk of being pedantic, you seem to be using big O to mean “approximately” or “in the order of”, but that’s not what it means at all. Big O is an expression of the growth rate of a function. Any constant value has a growth rate of 0, so O(100k) isn’t meaningful: It’s exactly the same as O(1).
