that's great news
This weekend I made some quick experiments with muPDF and poppler to measure how fast they render the pages. I've used pdfdraw(mupdf) and pdftoppm(poppler). In both cases modifications were made in order to measure only the rendering time and to exclude the time needed to write the image to the flash drive. It looks like muPDF is twice as fast as poppler. About the mupdf rendering problem that I've told you about in a previous(OT) post, it turns out to be introduced in a relatively recent commit and reverting to an older revision(28e88083d782eb) does work as it should. The people at Freenode's #ghostscript channel seem to be quite responsive too. So I completely agree with your choice of PDF rendering library.
I admit I know nothing about Lua, except what I am hearing about it - that it is minimalistic and fast(for a scripting language). Are you planning to use it just for a quick prototyping or you think it should be part of the end product too?
What library you plan to use for the UI? I still think Qt would be a great choice if we can make it fast enough. I still haven't have the time to play with it, but I intend to do it. In my opinion using a tested and mature UI toolkit could be a big advantage. I don't have a previous experience with Qt but I think it can be quicker and more pleasant if we are able to develop the application on a desktop using the virtual framebuffer features of Qt(qvfb).