The XMB scripts in forumdisplay.php and viewthread.php are the most performance critical. On an active public forum, those two scripts are continuously accessed by members, guests, and automated agents such as Googlebot.
One important consideration for webmasters is that XMB does not contain the code needed to optimize long-running threads. Some new tweaks were added in the eleventh patch to version 1.9.11 that will boost speed overall. However, performance is still expected to degrade whenever threads are allowed to grow much beyond 1000 replies. This is not a firm number, but testing has shown serious delays in database queries when threads reach 10^5 magnitude (100,000 replies or more). Threads with around 0 to 1000 replies are considered highly optimal. Support for very large threads may be added in a future version.
Public forums containing 10^5 threads (100,000 topics) are also a cause for concern because they may be accessed frequently during search engine crawls. Consider using sub-forums to organize any area of your board that contains more than 10000 threads. Each sub-forum is indexed separately and is optimized to display the most recent threads fastest.