The compilation should not restart “entirely”. One problem with webkit is the unified builds, this will cause unrelated sections to be rebuild even if you only work on one. The other is wether you used ninja or the build script directly.
You should also take care to have installed ccache before configuring the build, it will for the same inputs cache the result, and therefore be able to skip a lot of re-compilation that did not need to be done.
Another option for speeding up the build is using LLVM/Clang instead of gcc, especially using the llvm linker worked better iirc.
… All stuff we should write into the Readme 