Well, a lot of its speed it currently gains from a simple caching strategy. It will always render the next page just after displaying the current one.
I think starting on zooming and navigation will be the next thing I'll tackle. I will have to revise the caching mechanism and policy for that, I think. Also I'd like to include support for muPDFs "bbox" device that will calculate a "real usage" dependent bounding box for a given page which would allow for cut-to-content feature (the efficiency of which would depend on how the PDF is created, though).
Note that I just added an "emulator" mode that will use SDL for rendering/input, so development on a typical linux PC is possible. This allows for rapid prototyping.
Don't feel pressed to start on this. It's still a "pet project" for me, I'm doing it just for the joy of doing it. BTW, dpavlin already had a look at it and played with it. Github is a great help for integration work, so if you start playing with it, I suggest to just clone the Github repo.