If it's done right it would only require toggling a class on a main container (e.g. forum-list--row, forum-list--column), the appearance can all be changed with CSS. A simple class toggle in vanilla JS (As opposed to jQuery, which requires the whole library to load) at the bottom of the body wouldn't really have much of a speed difference.
This is just an example showing how it can be done easily with little performance impact. Further optimizations can be made using things like PHP (Though much more technical) and transitioning the switch between them so it isn't jarring.
/* Localstorage plugin would go here */
(function() {
var forumList = document.querySelector('.forum-list');
if(Storage.get('forum-orientation') == 'column') {
forumList.classList.remove('forum-list--row');
forumList.classList.add('forum-list--column');
}
});
This is just an example showing how it can be done easily with little performance impact. Further optimizations can be made using things like PHP (Though much more technical) and transitioning the switch between them so it isn't jarring.