Finish a feature branch and push it. Check out master. Pull master. Make new feature branch. Get an urgent bug fix request, switch to release branch, make a bugfix branch. Now go back to the new feature.
During this I may end up doing large rebuilds because Git changes modification times even if you end up back with the same header file you started with.
On a Thinkpad T540 the rebuild can take 30 minutes. It's less than 5 on a 3900X.
But this still results in slow compile times because the less used release directories get stale, have to be pulled up to date and often mostly rebuilt anyway. If I remember to actually use the second checkout directory, it does save on needing to rebuild the master/feature branches afterward.
In higher-level languages, running tests often takes considerably more time than building. A prosumer CPU like this would bring nice day to day improvements.
I have recently bought a 3900X. Some things about compilation I would note is that it heavily depends on your programming language and tooling. And that you would note is that you get most benefits on clean builds of big projects, which scale very well per core. On incremental builds or smaller projects it is however not uncommon to see < 25% of the CPU being used. That is especially with Rust, where a single compilation unit (crate) is compiled in a single-threaded fashion. Might be better with C or C++. But then again linking might also be blockers.
It's all nice and fast, but on day to day use you likely won't see a 100% speed increase compared to a 3600.