hi, i see what you are talking about, let me tell you, you don't need an "open source" CMS, instead you just need a good "light-weight" Framework to build your own custom CMS on top of it. a few good php framework options to use are, code-ignitor , laravel and zend.
The sites with high volume traffic which need to serve to a lot of users simultaneously, securely and efficiently, have to be planned very carefully in terms of architecture.
please let me know your current site. I can do a whole revamp of your site in order to make it stable for high volume usage.
i just signed NDA on your request, can you please provide more details now to analyze?
thank you and regards, adnan