You think one tiny column with a check mark in it would make this site slow?
Right now, to render a board page, the server hits the database to find the topics with the most recent timestamps. Depending on how it's coded, it also may or may not count the number of replies to each thread on the fly. The server compares each thread's timestamp vs. the last time you viewed the page and decides whether to put a "new" tag.
Your tiny column with a check mark alone would probably take about as much cpu time as everything already mentioned put together. Instead of just checking times and counting posts, the server would have to check whether any of the potentially thousands of posts were made by you.
Would it absolutely kill the site? No, but just a couple of tiny features like that will have a noticeable effect.