I was using linked-list to append things to the head, which was supposed to be it's optimal use case. I've refactored the function it's calling to take an n argument, which means to leave first n holes in the returned vector for the calling function to fill.

I had also used a linked list of characters to represent strings to iterate through them one by one. This was very slow compared to iterating through an array.

Linked-lists were supposed to be O(1) inserting at the head, and O(n) iterating through it. However, it's no match against array O(1) assignment and O(n) array iteration.