Competitive programming gives you some good intuition on these types of things. N of around 100000 is where you stop looking for O(n^2) algorithms, but the key

Competitive programming gives you some good intuition on these types of things. N of around 100000 is where you stop looking for O(n^2) algorithms, but the key there is you only have single-digit-seconds for your solution to run. Which is just to say: yeah, machines can chew through _a lot_ of crap, especially if it's not something a user has to wait for and you can do it over-night or something.