I don't have time to perform a review right now, I'm trying to get PEP 447 through review and that takes most of my available time at the moment.

I'm not convinced that the speedup of plistlib is relevant for real-world code, plist files are intended as simple configuration files and tend to contain little data and should be read/written only sporadically.

That said some people appear to abuse plistlib to process other files which are probably NSKeyedArchiver archives, and those can be a lot larger. But I'm opposed to explicitly supporting that use case, because the format of NSKeyedArchiver files is completely undocumented.