ccache eliminates spurious rebuilds, but doesn't make compilation of things which actually need to be recompiled any faster. It's basically just a workaround for that Make only uses file mtimes and not the contents to decide what needs to be built.

In theory, agreed. In practice I see a lot of re-compilation in large projects.

ccache doesn't help as soon as you change a popular header file. distcc helps quite a bit, but there was a project I worked on for years where by the end the compile time was 15 minutes after you'd distributed it across 20 machines.