On sites with over half a million comment posts (normally WSN Forum discussion forums, but potentially the other scripts as well), the process of rebuilding the table index after someone posts will make the post take a long time to submit, will spike the server load and will consequently slow down the whole site for a while. If you're seeing the load go up when people post while your server performs fine when nobody's posting, this may be your issue.

The solution to this issue is archiving inactive comment threads. Inactive threads will be locked so that nobody can reply to them, and their data will be moved to another database table in order to allow the regular comments table to become small enough to do fast insertions.

To begin archiving inactive threads, go to Admin Panel -> Listings -> Comment Settings -> "Archiving Old Threads" (requires WSN 9.0.5 or later). Specify your desired minimum number of days of inactivity. Do not set this too low, as it will not be possible to de-archive a thread once it's archived. Our recommendation is 365 days.

Once you've set the number of days, the process of archiving will begin automatically. If you need to speed it up, visit yoursite/index.php?action=archive and let that run to completion.