Strain slowing it down.
Every user has a certain number representing their name. Every thread has a number representing it and every post has a number representing it.
Very basically, threads are a collection of posts that match the thread number and shown in order of date posted.
The functionality here is to note down the number of every thread you post in. Then when you load certain pages showing the threads, replace the icon with an icon with a dot for any thread numbers that match the thread numbers that you have posted in.
However, as we have tens of thousands of threads and tens of thousands of users a record would have to be kept for every single user and every single thread and whether they combine. It's not as expensive a query as it sounds but implemented wrongly then it could hurt us server wise in terms of slowing down the site. We'll need to look into it and how best implemented in this version of the software but it is by no means impossible.