enigma1 - 3:25 pm on Oct 26, 2011 (gmt 0)
It's all matter of customizations no matter which cart you choose. The difference with proprietary vs open source carts is with the first type you get stuck with the company who released it. And they aren't MS. With the open source ones it's easier to find people who know the framework and customize it and often way cheaper.
I used osCommerce for years and tested it with 100's of thousands of products. Without customizations the cart will not function well but doing modifications it offers excellent performance. And it's tiny. That's another important factor. The more the bloat and code, the more you spend to customize it (effort, time, money etc).
And then it's the hosting. I have seen cases where merchants were on a dedicated server spending thousands annually and page loading was taking ages. So it's not only the server but the port the host allocates and how many other boxes go through it. You need to be on top of things all the time, change host or inform the host about problems when necessary.