It is all about processor power.
Thread/Forum Read Marking Type
1) Inactivity/Cookie Based - once a user has been inactive for a certain amount of time (the value of the cookie timeout option) all threads and forums are considered read. Individual threads are marked as read within a session via cookies. This option is how all versions of vBulletin before 3.5 functioned.
2) Database (no automatic forum marking) - this option uses the database to store thread and forum read times. This allows accurate read markers to be kept indefinitely. However, in order for a forum to be marked read when all threads are read, the user must view the list of threads for that forum. This option is more space and processor intensive than inactivity-based marking.
3) Database (automatic forum marking) - this option is the same as a previous option, but forums are automatically marked as read when the last new thread is read. This is the most usable option for end users, but most processor intensive.
# 1 Inactivity; I have yet to truly find how that one works, a lot of variables come into play.
# 3 would be the best for all; it is also has the most processor usage.
So with the version in use it could be changed but would use some Serious Horse Power with the number of users.
I had asked about this some time back. The other one was that if all the Political threads could be left out of the New Posts/Threads when the Button is clicked.
It is serious undertaking to run a board of this size.
Later AJ
