In my opinion it is a bad choice NOT to use existing framework (btw. XPages are a framework too) especially if you plan to run your applications in the WWW. First, there are a lot of security reasons (parameter handling, SQL injections, CSFR etc.) - actual frameworks have protection mechanisms implemented by default. And second, why reinvent the wheel?
A good example for creating a working application first and THEN optimize it, is Facebook itself. Beacuse that is exactly what happens here: if FB started, the technology used was total ignorable. If you look back on this decision today, it was a bad idea not to write everything in C++. But FB is now 10 years old, and they are changing this NOW* by switching the backend and/or implement new things in C++. Witch makes sense from the ROI point of view, because the investment of a million $ to save millions of $ (by reducing the server costs with lower hardware requirements) is a good choice.
(Don't forget that 'backend' also includes processes for their Big Data Analysis).
And this brings us back to my point of view: Your decision depends in your requirements. If you know you have to run your application for million of users from the first second, optimize it and write everything in C++. In this case, your budget shouldn't matter about the effort you will have (compared with a framework if your choice). The money you will save because of lower HW requirements will easily compensate it.
But if you have a startup with no/low budget, and don't know how many users you will have, try to implement it scalable as possible with the lowest effort. If your server is running on the limit, add a second, a third etc). If Yahoo, FB, Twitter or Google then are buying your company for 1 billion $, use the money to rewrite everything.
You should always think twice about the requirements you currently have to make a decision about the required technologies. But don't think about "maybe in five years this could bring some troubles" and make your life harder than it is.
Just my two cents....
P.S. What's your definition of a large XPages application?
What is your requirement / definition for your CMS? This is the main question before choosing a technology.
*: Now means since three or four years