Without getting too deep into technical specifics, Kaleidescape servers have databases and background processes that are run in order to maintain and serve up your movie library. Things like movie guide updates (the metadata about all of the movies on your system), library updates (what collections you have created, what movies are in which collection(s), what movies have been played, what movies have been paused (and where), downloading content, etc. are just a few examples of data that has to be updated and the processes being run.
This has to happen on each and every server that is part of your Kaleidescape system. In turn, those updates also need to be synced between every server in the system. As the number of servers in the system increases, so does the complexity of keeping the information on all of those servers in sync. In addition to this added complexity of syncing information, as the number of titles in your library grows, the amount of time it takes to process various background tasks begins to take longer.
So in your example above, a system comprised of a single 48 TB server would be more performant than a system comprised of two 22 TB servers (although you're still under the 4 server limit at this point so the performance difference would be negligible). It's true that the 48 TB server might encounter some performance hits as the library grows but the two server system would encounter those performance hits sooner since there are added processes running in order to keep the two servers' in sync.
Then you have to factor in the performance differences between server types. The Terra or Terra Prime has more processing power than a Strato and the Terra doesn't have to support video and audio codecs, manage a user interface, or actually play content. So when one of the servers in your system is actually a Strato or Alto, you've already introduced a system performance bottleneck since the background processes on the Strato are slower (both because of less processing power _and_ because first and foremost, the Strato needs to be able to play video so things like guide update processing or movie downloads have to take a backseat to actually playing video).
As for the Terra Prime SSDs, they are currently the most performant servers in the Terra line up. In addition to the improved processing power of the Terra Prime, they have the benefit of the faster (and more consistent) data read and write speeds of the SSDs. Whereas the read and write speeds on a HDD are quite dependent on where on the disc platter the data is located, there is no such performance penalty when accessing an SSD. This is what enables those servers to be able to serve 25 simultaneous playback zones as compared to the 10 supported by Terra's with HDDs in them and allows the faster download speeds.