Actually on busy boards the 2 queries used to generate that information always took the longest and was the main cause for slowdowns on these boards as it would do a full table scan, twice. If you really want to make it less than 15 you can edit a couple files to reduce it.
Line 161 of admin/doeditconfig.php and line 680 of ubbthreads.php. Change 15 to something less.
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