UBB scales as well as the server allows it to - right now, the UBB bottleneck is in the I/O. If the UBB can not get to the data files it requires fast enough, it will begin to slow things down. What looks like too much CPU use is in reality too much I/O. We're taking measures to reduce the UBB's I/O dependance in future releases.
In the mean time, please ask your host for some stats on their server - i.e. what OS, processor, memory, I/O subsystem they're using... I'd like to know.
