The performance of a database backed www-system like CB is hevily dependent on the performance of the database and queries.
Properly indexed database performance does not degrade linearly when data increases. On a moder hardware, MySQL can handle millions of rows of data with reasonable speed if the queries are as simple as they are in this application. The first thing to slow down is the connections.
The limiting factor is how many page views per second you get, not the size of the community. And of course your harware.
There are most likely administrative or social problems in the community before first signs of technical problems occur.
As more plugins plugins are installed more queries are needed putting a heavier load on the cpu..
My site has cpu limitation problems running dedicated on 2.8mhz and 1 gig memory having only 30 users and 50 guests.
Things like the connection path where lotsa data needs to be referenced seems to be a littlebit of a load increase and i had to turn it off.. basicaly any extras you install or use will put a higher load on your cpu..
stay away from the zoom gallery, it is definately not scalable and is my main concern on my website...
I'm not sure but i noticed that the connection tab lists all the friends of a user. What if the person will have hundreds of friends. Will it still make a long list all of them, or will it create a paginated list of friends under the connections tab?
I'm ALMOST sure it paginates. If you go to CB Lists here at Joomlapolis and select all users, that paginates. So it's something the core developers were well aware of.