About porting the project to Qt, one issue I have with Qt is that it's a bit of a monster (I mean it's huge) haha. I do not code native C++ anymore, but when I did I used to use FLTK as a small and lightweight portable GUI framework. It's maybe not as fancy as Qt, but it not bad. Other alternatives (what I do now), is just to use Java or Javascript/html5 and forget about the portability nightmare 
[and btw, about compile times: the 6x in particular is basically building MDL, passing all the unit tests, some of which take a while to run, and takes 6x longer on my previous intel laptop, but there's lot of parallelism in running unit tests, so, probably that build is benefitting from that]
I agree that if you simply need a GUI framework, Qt is heavy. But to be fair, Qt is much more than that - a very complete library covering many portability issues such as threads, processes and file system access, just to name a few. And not to forget about the tools: a GUI Designer, Qt Creator and build system (ignoring the Qbs desaster ;-) ), which makes building the same project on different platforms a smooth experience.
When it comes to just porting an existing application, for the sake of efficiency, it's easier for me to stick to tools and frameworks that I already feel comfortable with, and of course to the language of the original application.
Speaking of size, you have to admit that neither Java nor Browsers can be considered "lightweight" 
about the lightweight, hahaha, true, good point 