Why O(n) Was Slower Than O(n log n) - And It Broke Everything I Thought I Knew About Time Complexity

for example: arr1 = [1, 3, 2, 1, 4, 5] arr2 = [3, 2, 2, 1, 1] answer = [3, 2, 1] simple enough right? and if you've done even a little bit of DSA prep your brain immediately goes - HashSet. put all e