Hello,
yes, in a long term, if the autogenerated URLs are not purged from time to time, the number of links (especialy if your forums are often visited) may grow very high and may become a performance issue. How much depends only on your HW and SW configuration.
We try to optimize the components as much as possible, but there is always decision between usablity/performance. In this case, most of the links need to be written to DB and of course that with increasing number of links, the DB load will be increasing. In case of the forums, the easiest solution may be to purge the generated URLs reguralily to get rid of very old, not anymore visited links.
Best regards,
Michal