If you want more details: Lots of parts are obviously pure C, but in my experience the C++ stuff is usually quite incremental as well. Changing a header included in lots of places is the obvious multiplier to build time.

Linking does take some time, but it’s only really noticeable if you do very incremental changes (e.g. change a single value, build, boot, repeat).

Overall, building a whole kernel from scratch even for multiple configurations is not unbearable during normal development.