There is no need for zip extraction to be real time so i don't see why it would be a problem that the cpu speed in most targets is not that big. But as you say it will probably be very slow :-)
I'm not sure what you mean by "real time" when dealing with zip files...
But a zip/unzip implementation in Rockbox is very feasible - the code from the infozip project is extremely portable, and even runs on old 16-bit MS-DOS systems. ZIP is an old format - meaning it was designed to run on computers with less horsepower than the modern targets Rockbox runs on.
I also think it could be quite useful - the unzip functionality could be added to the plugin library (thereby making it usable by all plugins, such as the various emulators, enabling them to read data directly from zip files without uncompressing them first), as well as a general-purpose unzip plugin.
The fact that people have long been requesting such a plugin shows that at least those people would have a use for it.