All the Differential/code review builds currently clog our Jenkins build queue. In particular for Windows we have a single executor for all MITK build jobs which leads to the situation, that even the nightly build will probably be delayed tonight. On the other side we already experienced that it is not a good idea to do Differential builds in continuous/incremental mode, as it will require manual interference now and then on the build clients.
However, I think it should be possible to write a script that checks if anything in CMakeExternals/ changed compared to the last Differential build. If this is the case, do a clean build, but otherwise just delete MITK-build instead of the whole workspace. This would already save as many hours of build time.