I'm assuming that you're referencing UBB.Classic, which was coded in Perl and stored all activities in flat files, versus UBB.threads
which was/is coded in PHP and activities are stored in a database (MySQL).
The shortcomings of UBB.Classic was that, since storage was done in flat files, the system would get lagged from all of the read/write activity to the local hard disk's; this was especially bad for more popular sites (as more activity means more io/wait).
At one point, a small organization was formed with the goal of developing a PHP port of UBB.Classic; this group is known today as vBulletin. The fun part to think about is, they tried to sell their product to Infopop, which wasn't interested since they acquired WWWThreads, which they renamed to UBB.threads