(I have replied on 27.02.2013 to an exactly same question in our contact form. So if it's you, please check your spam and trash for the detailed reply. Here a detailed copy of it so that it's also publicly available):
Hi Jon,
CB and its CB Team add-ons are very well scalable, and basically similarly scalable as Joomla itself.
I have seen Joomla+CB powered sites with thousands or dozens of thousands users online.
We have done optimizations in order for such active sites to work well.
However, the limiting factor is usually not CB itself, but everything around it: e.g.
- other extensions
- a single small non-optimized third-party module, plugin or component
- the hosting server itself
- the settings of the server and of the webserver, PHP and Mysql database
- the network connection
can ruin your whole server performance
Now speaking of critical high volume sites with more than hundreds or thousands of users online:
Independently of CB, you need to be aware that running such high-traffic sites requires a good hosting engineer at hand or in the team at least at set-up stage.
There are al lots of web-server and database-server fine-tunings to do so that the system itself can sustain such traffic.
You also need to avoid or optimize third-party extensions which are not well designed of course.
Finally, depending on your settings, you might need to add database indexes to optimize database load.
The above is true whatever software you choose on a high-traffic site.
All of that requires to have a skilled engineer at hand or in team.
To start I would recommend to start by avoiding Joomla 1.5, and going with Joomla 3.0 preferably, as you will gain 30%+ performance in Joomla itself. I would also recommend to start with a well-optimized server, with root access for the optimization expert(s) in case they need to monitor or debug performance.
We also have consulting services by the hour in menu Support/services tab to help you if needed (except for third-party items), as we are always interested in high-traffic CB sites. But those available hours are limited, so you will still need your own engineer if you really want to go high in online users. We also have shared hosting services on high-performance high-availability redundant, higly-optimized servers similar to the one hosting this site (Hosting menu at top).
Best Regards,