We generate pchs for every header file outside of a project part. So the pchs contains header files from other project parts. This can lead to an expensive recreation of many pchs if a central header is changed. And the generation is very expensive. But we can reduce this expense if we split the pchs in two parts. One contains the headers from outside the project like Qt and the STL and the other contains header from the other project parts. So we need only recreate the second pchs which is quite cheaper.
This can be also be combined with merging of project parts.