ccache works at the translation unit level, caching object files. Zapcc caches (among other things) template instantiations between compilations.
If you are in a codebase that has some very complex header files with lots of (potentially nested) template instantiations, and then a large number of executables that all instantiate these templates, Zapcc can make a huge difference in compilation times. CCache doesn't really buy you anything in this scenario.
zapcc speeds up the build even if you changed a #define across the whole project or added a line to a header that you use everywhere leading to recompiling of all your .cpp files.
