just as a benchmark -- i support ( make mods and do other stuff for the client ) 2 forums that run on the same box
1 forum is 9million posts
other forum is 7million posts
server load never goes over 2, which really means it's twiddling it's thumbs.
server just loaded up on RAM ( 32gig ) and SSDs ( RAID ) for the OS / data.
so UBB is very efficient
