I am coding an interactive epub for iBooks using zepto.js, plus a few of my own scripts.
As I understand it, it's best practice to combine javascript files for improved performance (as you are reducing http requests to the server).
Is there a performance advantage if your files are local?
To my knowledge, there's no way of benchmarking javascript performance in iBooks; however, I couldn't detect a reduction in performance when loading separate javascript files. And, if anything, performance appeared to get worse when I combined my javascript files.
So, it seems that there is no performance advantage to combining local javascript files in iBooks.