Why does adding elements to a set take longer than adding elements to a list in python? I created a loop and iterated over 1000000 elements added it to a list and a set. List is consistently taking around 10 seconds vs set which is taking around 20 seconds.

Both operations are O(1) amortized time-complexity.

Appending elements to a list has a lower coefficient because it does not need to hash the element first, nor does it need to check/handle hash collisions.

In the case of adding x into a set, Python needs to compute hash(x) first, because keeping the hash of all elements is what allows sets to have fast O(1) membership checks (compared to O(n) membership checks for lists).