I use ZoneMinder for various reasons, but mostly because it’s free and from my point of view the only free open source system for video surveillance which is worth your time and money if you donate.
I would like to share with you my experience.How I got a significant low CPU usage from a third party JPEGlibrary, which I use on my CentOS5 installation with ZoneMinder.This library is well written to optimize the JPEG processing power of the CPU.
Just recently I had to install Elastix on OpenVZ. This post on the Elastixforum will help you to install DAHDI. The idea is very simple, as every complex problem has simple and elegant solution, and I can confirm you that from my years of experience. I will give two words Physicalto Virtual, and I hope this ring some bells :), here is what you have to do.