ccache speeds up compilation of single files by quite a lot, by effectively avoiding unnecessary recompilation. There are distributed caches that work like ccache too. Compiling a file for a second or fiftieth time with no changes because of doing a clean build is the absolute most common case. Maybe zapcc does additional caching of compiler internal state, but I would have to look into it to see if it's actually good enough. Also, ccache works with many compilers, whereas zapcc is its own compiler. That is a huge advantage.
ccache does not speed up compilation at all, in fact it slows it down. It only speeds up re-compilation of the same translation unit (as in, bitwise identical preprocessed source code) which is often not all that useful, especially when the local development rebuild use case is already covered by make.
ccache hardly slows anything down. It is a thin wrapper around running the compiler. You seem to kind of understand how it works, but it has multiple configurable ways to detect whether a file should be compiled. It does a LOT more than make, which does NOTHING to handle clean rebuilds or multiple builds of similar code in different locations. Unlike make, it does not rely on file timestamps alone to decide whether to rebuild an output.
When you do a clean build, it will pull that output from the cache if it has been compiled before and has not changed since. It does not technically speed up compilation of files. It bypasses compilation when it can.
