[eluser]Ty Bex[/eluser]
From my experience, CI should not have much of direct effect on performance or capacity.
Performance and capacity are a direct relation to Custom code, Web Content, Number of users, Activity of users and hardware.
There are a number of methods that you can use to improve performance with CI, Code and Conent and if all else fails then it may just be a hardware scaling issue.
If you run into "SLOWNESS" issues it comes down find the root cause.
I am about to launch CI on a server that has a couple thousand unique visitors a day with tens of thousands of page hits per day. This is on a dedicated server that is 2 years old. I am not concerned about CI causing issues, I am more concerned about the size of the content that I am displaying to the user.