Given that the use case is "protect my biology homework from my little brother", how fast does the implementation really need to be? Is speeding it up from 0.1 seconds to 0.001 seconds worth the potential new problems that come with more C code (more code to maintain, less portability to other runtimes, potential for interpreter crashes or even arbitrary code execution vulnerabilities from specially crafted files)?
Also, if the use case is just protecting stuff from a sibling or your childen, use an archiving program to zip/extract it :)
So -1 here as well. Any added C code has a real cost for the reasons Jean-Paul listed, so it should only be used in cases where there's a major practical benefit to the speed-up. Faster execution of a problematic algorithm that is already well implemented by plenty of other applications doesn't qualify in my book (even if the speedup is by a couple of orders of magnitude).